javascript extract the assignment of from this expression

Destructuring assignment

The two most used data structures in JavaScript are Object and Array .

• Objects allow us to create a single entity that stores data items by key.
• Arrays allow us to gather data items into an ordered list.

However, when we pass these to a function, we may not need all of it. The function might only require certain elements or properties.

Destructuring assignment is a special syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more convenient.

Destructuring also works well with complex functions that have a lot of parameters, default values, and so on. Soon we’ll see that.

Array destructuring

Here’s an example of how an array is destructured into variables:

Now we can work with variables instead of array members.

It looks great when combined with split or other array-returning methods:

As you can see, the syntax is simple. There are several peculiar details though. Let’s see more examples to understand it better.

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. However, the array itself is not modified.

It’s just a shorter way to write:

Unwanted elements of the array can also be thrown away via an extra comma:

In the code above, the second element of the array is skipped, the third one is assigned to title , and the rest of the array items are also skipped (as there are no variables for them).

…Actually, we can use it with any iterable, not only arrays:

That works, because internally a destructuring assignment works by iterating over the right value. It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values.

We can use any “assignables” on the left side.

For instance, an object property:

In the previous chapter, we saw the Object.entries(obj) method.

We can use it with destructuring to loop over the keys-and-values of an object:

The similar code for a Map is simpler, as it’s iterable:

There’s a well-known trick for swapping values of two variables using a destructuring assignment:

Here we create a temporary array of two variables and immediately destructure it in swapped order.

We can swap more than two variables this way.

The rest ‘…’

Usually, if the array is longer than the list at the left, the “extra” items are omitted.

For example, here only two items are taken, and the rest is just ignored:

If we’d like also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "..." :

The value of rest is the array of the remaining array elements.

We can use any other variable name in place of rest , just make sure it has three dots before it and goes last in the destructuring assignment.

Default values

If the array is shorter than the list of variables on the left, there will be no errors. Absent values are considered undefined:

If we want a “default” value to replace the missing one, we can provide it using = :

Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided.

For instance, here we use the prompt function for two defaults:

Please note: the prompt will run only for the missing value ( surname ).

Object destructuring

The destructuring assignment also works with objects.

The basic syntax is:

We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like “pattern” for corresponding properties. In the simplest case, that’s a list of variable names in {...} .

For instance:

Properties options.title , options.width and options.height are assigned to the corresponding variables.

The order does not matter. This works too:

The pattern on the left side may be more complex and specify the mapping between properties and variables.

If we want to assign a property to a variable with another name, for instance, make options.width go into the variable named w , then we can set the variable name using a colon:

The colon shows “what : goes where”. In the example above the property width goes to w , property height goes to h , and title is assigned to the same name.

For potentially missing properties we can set default values using "=" , like this:

Just like with arrays or function parameters, default values can be any expressions or even function calls. They will be evaluated if the value is not provided.

In the code below prompt asks for width , but not for title :

We also can combine both the colon and equality:

If we have a complex object with many properties, we can extract only what we need:

The rest pattern “…”

What if the object has more properties than we have variables? Can we take some and then assign the “rest” somewhere?

We can use the rest pattern, just like we did with arrays. It’s not supported by some older browsers (IE, use Babel to polyfill it), but works in modern ones.

It looks like this:

In the examples above variables were declared right in the assignment: let {…} = {…} . Of course, we could use existing variables too, without let . But there’s a catch.

This won’t work:

The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. Such code blocks can be used to group statements, like this:

So here JavaScript assumes that we have a code block, that’s why there’s an error. We want destructuring instead.

To show JavaScript that it’s not a code block, we can wrap the expression in parentheses (...) :

Nested destructuring

If an object or an array contains other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.

In the code below options has another object in the property size and an array in the property items . The pattern on the left side of the assignment has the same structure to extract values from them:

All properties of options object except extra which is absent in the left part, are assigned to corresponding variables:

Finally, we have width , height , item1 , item2 and title from the default value.

Note that there are no variables for size and items , as we take their content instead.

Smart function parameters

There are times when a function has many parameters, most of which are optional. That’s especially true for user interfaces. Imagine a function that creates a menu. It may have a width, a height, a title, an item list and so on.

Here’s a bad way to write such a function:

In real-life, the problem is how to remember the order of arguments. Usually, IDEs try to help us, especially if the code is well-documented, but still… Another problem is how to call a function when most parameters are ok by default.

That’s ugly. And becomes unreadable when we deal with more parameters.

Destructuring comes to the rescue!

We can pass parameters as an object, and the function immediately destructurizes them into variables:

We can also use more complex destructuring with nested objects and colon mappings:

The full syntax is the same as for a destructuring assignment:

Then, for an object of parameters, there will be a variable varName for the property incomingProperty , with defaultValue by default.

Please note that such destructuring assumes that showMenu() does have an argument. If we want all values by default, then we should specify an empty object:

We can fix this by making {} the default value for the whole object of parameters:

In the code above, the whole arguments object is {} by default, so there’s always something to destructurize.

Destructuring assignment allows for instantly mapping an object or array onto many variables.

The full object syntax:

This means that property prop should go into the variable varName and, if no such property exists, then the default value should be used.

Object properties that have no mapping are copied to the rest object.

The full array syntax:

The first item goes to item1 ; the second goes into item2 , and all the rest makes the array rest .

It’s possible to extract data from nested arrays/objects, for that the left side must have the same structure as the right one.

We have an object:

Write the destructuring assignment that reads:

• name property into the variable name .
• years property into the variable age .
• isAdmin property into the variable isAdmin (false, if no such property)

Here’s an example of the values after your assignment:

The maximal salary

There is a salaries object:

Create the function topSalary(salaries) that returns the name of the top-paid person.

• If salaries is empty, it should return null .
• If there are multiple top-paid persons, return any of them.

P.S. Use Object.entries and destructuring to iterate over key/value pairs.

Open a sandbox with tests.

Open the solution with tests in a sandbox.

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Left-hand-side expressions

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This chapter describes JavaScript's expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, ternary and more.

A complete and detailed list of operators and expressions is also available in the reference .

JavaScript has the following types of operators. This section describes the operators and contains information about operator precedence.

• Assignment operators
• Comparison operators
• Arithmetic operators
• Bitwise operators

Logical operators

String operators, conditional (ternary) operator.

• Comma operator

Unary operators

• Relational operator

JavaScript has both binary and unary operators, and one special ternary operator, the conditional operator. A binary operator requires two operands, one before the operator and one after the operator:

For example, 3+4 or x*y .

A unary operator requires a single operand, either before or after the operator:

For example, x++ or ++x .

An assignment operator assigns a value to its left operand based on the value of its right operand. The simple assignment operator is equal ( = ), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x .

There are also compound assignment operators that are shorthand for the operations listed in the following table:

Destructuring

For more complex assignments, the destructuring assignment syntax is a JavaScript expression that makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.

A comparison operator compares its operands and returns a logical value based on whether the comparison is true. The operands can be numerical, string, logical, or object values. Strings are compared based on standard lexicographical ordering, using Unicode values. In most cases, if the two operands are not of the same type, JavaScript attempts to convert them to an appropriate type for the comparison. This behavior generally results in comparing the operands numerically. The sole exceptions to type conversion within comparisons involve the === and !== operators, which perform strict equality and inequality comparisons. These operators do not attempt to convert the operands to compatible types before checking equality. The following table describes the comparison operators in terms of this sample code:

Note:  ( => ) is not an operator, but the notation for Arrow functions .

An arithmetic operator takes numerical values (either literals or variables) as their operands and returns a single numerical value. The standard arithmetic operators are addition ( + ), subtraction ( - ), multiplication ( * ), and division ( / ). These operators work as they do in most other programming languages when used with floating point numbers (in particular, note that division by zero produces Infinity ). For example:

In addition to the standard arithmetic operations (+, -, * /), JavaScript provides the arithmetic operators listed in the following table:

A bitwise operator treats their operands as a set of 32 bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.

The following table summarizes JavaScript's bitwise operators.

Bitwise logical operators

Conceptually, the bitwise logical operators work as follows:

• The operands are converted to thirty-two-bit integers and expressed by a series of bits (zeros and ones). Numbers with more than 32 bits get their most significant bits discarded. For example, the following integer with more than 32 bits will be converted to a 32 bit integer: Before: 11100110111110100000000000000110000000000001 After: 10100000000000000110000000000001
• Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.
• The operator is applied to each pair of bits, and the result is constructed bitwise.

For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:

Note that all 32 bits are inverted using the Bitwise NOT operator, and that values with the most significant (left-most) bit set to 1 represent negative numbers (two's-complement representation).

Bitwise shift operators

The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operand.

The shift operators are listed in the following table.

Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.

Examples of expressions that can be converted to false are those that evaluate to null, 0, NaN, the empty string (""), or undefined.

The following code shows examples of the && (logical AND) operator.

The following code shows examples of the || (logical OR) operator.

The following code shows examples of the ! (logical NOT) operator.

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

• false && anything is short-circuit evaluated to false.
• true || anything is short-circuit evaluated to true.

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings.

For example,

The shorthand assignment operator += can also be used to concatenate strings.

The conditional operator is the only JavaScript operator that takes three operands. The operator can have one of two values based on a condition. The syntax is:

If condition is true, the operator has the value of val1 . Otherwise it has the value of val2 . You can use the conditional operator anywhere you would use a standard operator.

This statement assigns the value "adult" to the variable status if age is eighteen or more. Otherwise, it assigns the value "minor" to status .

The comma operator ( , ) simply evaluates both of its operands and returns the value of the last operand. This operator is primarily used inside a for loop, to allow multiple variables to be updated each time through the loop.

For example, if a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to update two variables at once. The code prints the values of the diagonal elements in the array:

A unary operation is an operation with only one operand.

The delete operator deletes an object, an object's property, or an element at a specified index in an array. The syntax is:

where objectName is the name of an object, property is an existing property, and index is an integer representing the location of an element in an array.

The fourth form is legal only within a with statement, to delete a property from an object.

You can use the delete operator to delete variables declared implicitly but not those declared with the var statement.

If the delete operator succeeds, it sets the property or element to undefined . The delete operator returns true if the operation is possible; it returns false if the operation is not possible.

Deleting array elements

When you delete an array element, the array length is not affected. For example, if you delete a[3] , a[4] is still a[4] and a[3] is undefined.

When the delete operator removes an array element, that element is no longer in the array. In the following example, trees[3] is removed with delete . However, trees[3] is still addressable and returns undefined .

If you want an array element to exist but have an undefined value, use the undefined keyword instead of the delete operator. In the following example, trees[3] is assigned the value undefined , but the array element still exists:

The typeof operator is used in either of the following ways:

The typeof operator returns a string indicating the type of the unevaluated operand. operand is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.

Suppose you define the following variables:

The typeof operator returns the following results for these variables:

For the keywords true and null , the typeof operator returns the following results:

For a number or string, the typeof operator returns the following results:

For property values, the typeof operator returns the type of value the property contains:

For methods and functions, the typeof operator returns results as follows:

For predefined objects, the typeof operator returns results as follows:

The void operator is used in either of the following ways:

The void operator specifies an expression to be evaluated without returning a value. expression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.

You can use the void operator to specify an expression as a hypertext link. The expression is evaluated but is not loaded in place of the current document.

The following code creates a hypertext link that does nothing when the user clicks it. When the user clicks the link, void(0) evaluates to undefined , which has no effect in JavaScript.

The following code creates a hypertext link that submits a form when the user clicks it.

Relational operators

A relational operator compares its operands and returns a Boolean value based on whether the comparison is true.

The in operator returns true if the specified property is in the specified object. The syntax is:

where propNameOrNumber is a string or numeric expression representing a property name or array index, and objectName is the name of an object.

The following examples show some uses of the in operator.

The instanceof operator returns true if the specified object is of the specified object type. The syntax is:

where objectName is the name of the object to compare to objectType , and objectType is an object type, such as Date or Array .

Use instanceof when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exception-handling code depending on the type of exception thrown.

For example, the following code uses instanceof to determine whether theDay is a Date object. Because theDay is a Date object, the statements in the if statement execute.

The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.

The following table describes the precedence of operators, from highest to lowest.

A more detailed version of this table, complete with links to additional details about each operator, may be found in JavaScript Reference .

• Expressions

An expression is any valid unit of code that resolves to a value.

Every syntactically valid expression resolves to some value but conceptually, there are two types of expressions: with side effects (for example: those that assign value to a variable) and those that in some sense evaluates and therefore resolves to value.

The expression x = 7 is an example of the first type. This expression uses the = operator to assign the value seven to the variable x . The expression itself evaluates to seven.

The code 3 + 4 is an example of the second expression type. This expression uses the + operator to add three and four together without assigning the result, seven, to a variable. JavaScript has the following expression categories:

• Arithmetic: evaluates to a number, for example 3.14159. (Generally uses arithmetic operators .)
• String: evaluates to a character string, for example, "Fred" or "234". (Generally uses string operators .)
• Logical: evaluates to true or false. (Often involves logical operators .)
• Primary expressions: Basic keywords and general expressions in JavaScript.
• Left-hand-side expressions: Left values are the destination of an assignment.

Primary expressions

Basic keywords and general expressions in JavaScript.

Use the this keyword to refer to the current object. In general, this refers to the calling object in a method. Use this either with the dot or the bracket notation:

Suppose a function called validate validates an object's value property, given the object and the high and low values:

You could call validate in each form element's onChange event handler, using this to pass it the form element, as in the following example:

• Grouping operator

The grouping operator ( ) controls the precedence of evaluation in expressions. For example, you can override multiplication and division first, then addition and subtraction to evaluate addition first.

Comprehensions

Comprehensions are an experimental JavaScript feature, targeted to be included in a future ECMAScript version. There are two versions of comprehensions:

Comprehensions exist in many programming languages and allow you to quickly assemble a new array based on an existing one, for example.

Left values are the destination of an assignment.

You can use the new operator to create an instance of a user-defined object type or of one of the built-in object types. Use new as follows:

The super keyword is used to call functions on an object's parent. It is useful with classes to call the parent constructor, for example.

Spread operator

The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected.

Example: Today if you have an array and want to create a new array with the existing one being part of it, the array literal syntax is no longer sufficient and you have to fall back to imperative code, using a combination of push , splice , concat , etc. With spread syntax this becomes much more succinct:

Similarly, the spread operator works with function calls:

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Assignment Expressions

JavaScript uses the = operator to assign a value to a variable or property. For example:

The = operator expects its left-side operand to be an lvalue: a variable or object property (or array element). It expects its right-side operand to be an arbitrary value of any type. The value of an assignment expression is the value of the right-side operand. As a side effect, the = operator assigns the value on the right to the variable or property on the left so that future references to the variable or property evaluate to the value.

Although assignment expressions are usually quite simple, you may sometimes see the value of an assignment expression used as part of a larger expression. For example, you can assign and test a value in the same expression with code like this:

If you do this, be sure you are clear on the difference between the = and == operators! Note that = has very low precedence and parentheses are usually necessary when the value of an assignment is to be used in a larger expression.

The assignment operator has right-to-left associativity, which means that when multiple assignment operators appear in an expression, they are evaluated from right to left. Thus, you can write code like this to assign a single value to multiple variables:

Assignment with Operation

Besides the normal = assignment operator, JavaScript supports a number of other assignment operators that provide shortcuts by combining assignment with some other operation. For example, the += operator performs addition and assignment. The following expression:

is equivalent to this one:

As you might expect, the += operator works for numbers or strings. For numeric operands, it performs addition and assignment; for string operands, it performs concatenation and assignment.

Similar operators include -= , *= , &= , and so on. Table 4-3 lists them all.

Table 4-3. Assignment operators

In most cases, the expression:

where op is an operator, is equivalent to the expression:

In the first line, the expression a is evaluated once. In the second it is evaluated twice. The two cases will differ only if a includes side effects such as a function call or an increment operator. The following two assignments, for example, are not the same:

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Destructuring Assignment in JavaScript

Destructuring Assignment is a JavaScript expression that allows to unpack of values from arrays, or properties from objects, into distinct variables data can be extracted from arrays, objects, and nested objects, and assigned to variables . In Destructuring Assignment on the left-hand side, we define which value should be unpacked from the sourced variable. In general way implementation of the extraction of the array is as shown below: 

Example:  

• Array destructuring:
• Object destructuring:

Array destructuring

Using the Destructuring Assignment in JavaScript array possible situations, all the examples are listed below:

Example 1: When using destructuring assignment the same extraction can be done using below implementations. 

Example 2: The array elements can be skipped as well using a comma separator. A single comma can be used to skip a single array element. One key difference between the spread operator and array destructuring is that the spread operator unpacks all array elements into a comma-separated list which does not allow us to pick or choose which elements we want to assign to variables. To skip the whole array it can be done using the number of commas as there is a number of array elements. 

Example 3: In order to assign some array elements to variable and rest of the array elements to only a single variable can be achieved by using rest operator (…) as in below implementation. But one limitation of rest operator is that it works correctly only with the last elements implying a subarray cannot be obtained leaving the last element in the array. 

Example 4: Values can also be swapped using destructuring assignment as below: 

Example 5: Data can also be extracted from an array returned from a function. One advantage of using a destructuring assignment is that there is no need to manipulate an entire object in a function but just the fields that are required can be copied inside the function. 

Example 6: In ES5 to assign variables from objects its implementation is 

Example 7: The above implementation in ES6 using destructuring assignment is. 

Object destructuring

Example 1: The Nested objects can also be destructured using destructuring syntax. 

Example 2: Nested objects can also be destructuring

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• Destructuring assignment

Pulling values from a regular expression match

Es6 version, computed object property names and destructuring, firefox-specific notes.

This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal. Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

The destructuring assignment syntax is a JavaScript expression that makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.

{a, b} = {a:1, b:2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal.

However, the  ({a, b} = {a:1, b:2})  form is valid, as is the var {a, b} = {a:1, b:2} form.

Description

The object and array literal expressions provide an easy way to create ad hoc packages of data. Once you've created these packages of data, you can use them any way you want to. You can even return them from functions.

One particularly useful thing you can do with destructuring assignment is to read an entire structure in a single statement, although there are a number of interesting things you can do with them, as shown in the section full of examples that follows.

This capability is similar to features present in languages such as Perl and Python.

Array destructuring

Simple example, assignment without declaration.

Destructuring assignment can be made without a declaration in the assignment statement.

Swapping variables

After executing this code, b is 1 and a is 3. Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).

Multiple-value returns

Thanks to destructuring assignment, functions can return multiple values. While it's always been possible to return an array from a function, this provides an added degree of flexibility.

As you can see, returning results is done using an array-like notation, with all the values to return enclosed in brackets. You can return any number of results in this way. In this example, f() returns the values [1, 2] as its output.

The statement [a, b] = f() assigns the results of the function to the variables in brackets, in order: a is set to 1 and b is set to 2.

You can also retrieve the return values as an array:

In this case, a is an array containing the values 1 and 2.

Ignoring some returned values

You can also ignore return values that you're not interested in:

After running this code, a is 1 and b is 3. The value 2 is ignored. You can ignore any (or all) returned values this way. For example:

When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to pull the parts out of this array easily, ignoring the full match if it is not needed.

Object destructuring

The ( .. ) around the assignment statement is required syntax when using object literal destructuring assignment without a declaration.

Function argument defaults

Es5 version.

In Firefox, default values for destructuring assignments are not yet implemented: var { x = 3 } = {} and var [foo = "bar"] = []. See bug 932080 and also bug 1018628 for destructured default values in functions.

Module (non-ES6) loading

Destructuring can help to load specific subsets of a non-ES6 module like here in the Add-on SDK :

Nested object and array destructuring

For of iteration and destructuring, pulling fields from objects passed as function parameter.

This pulls the id , displayName and firstName from the user object and prints them.

Computed property names, like on object literals , can be used with destructuring.

Specifications

Browser compatibility

• Firefox provided a non-standard language extension in JS1.7 for destructuring. This extension has been removed in Gecko 40 (Firefox 40 / Thunderbird 40 / SeaMonkey 2.37). See bug 1083498 .
• Assignment operators

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Extract the assignment of from this expression with sonarQube

when I launch a sonar control I find myself with a smells code for the following function:

I get the following sonar alert:

Extract the assignment of “item.media” from this expression.

What would be the solution to avoid this sonar message?

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Destructuring assignment

The destructuring assignment syntax is a JavaScript expression that makes it possible to extract data from arrays or objects into distinct variables.

Description

The object and array literal expressions provide an easy way to create ad hoc packages of data.

The destructuring assignment uses similar syntax, but on the left-hand side of the assignment to define what elements to extract from the sourced variable.

This capability is similar to features present in languages such as Perl and Python.

Array destructuring

Basic variable assignment, assignment separate from declaration.

A variable can be assigned its value via destructuring separate from the variable's declaration.

Default values

A variable can be assigned a default, in the case that the value pulled from the array is undefined .

Swapping variables

Two variables values can be swapped in one destructuring expression.

Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).

Parsing an array returned from a function

It's always been possible to return an array from a function. Destructuring can make working with an array return value more concise.

In this example, f() returns the values [1, 2] as its output, which can be parsed in a single line with destructuring.

Ignoring some returned values

You can ignore return values that you're not interested in:

You can also ignore all returned values:

Pulling values from a regular expression match

When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to pull the parts out of this array easily, ignoring the full match if it is not needed.

Object destructuring

Basic assignment, assignment without declaration.

A variable can be assigned its value with destructuring separate from its declaration.

The ( .. ) around the assignment statement is required syntax when using object literal destructuring assignment without a declaration.

{a, b} = {a:1, b:2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal.

However, ({a, b} = {a:1, b:2})  is valid, as is  var {a, b} = {a:1, b:2}

Assigning to new variable names

A variable can be extracted from an object and assigned to a variable with a different name than the object property.

A variable can be assigned a default, in the case that the value pulled from the object is undefined .

Setting a function parameter's default value

Es5 version, es6 version.

In Firefox, default values for destructuring assignments are not yet implemented: var { x = 3 } = {} and var [foo = "bar"] = []. See bug 932080 for destructured default values in functions.

Module (non-ES6) loading

Destructuring can help to load specific subsets of a non-ES6 module like here in the Add-on SDK :

The import statement in ES6 has a superficial resemblance to destructuring, but is actually completely unrelated.

Nested object and array destructuring

For of iteration and destructuring, pulling fields from objects passed as function parameter.

This pulls the id , displayName and firstName from the user object and prints them.

Computed object property names and destructuring

Computed property names, like on object literals , can be used with destructuring.

Specifications

Browser compatibility

[1] Requires "Enable experimental Javascript features" to be enabled under `about:flags`

Firefox-specific notes

• Firefox provided a non-standard language extension in JS1.7 for destructuring. This extension has been removed in Gecko 40 (Firefox 40 / Thunderbird 40 / SeaMonkey 2.37). See bug 1083498 .
• Starting with Gecko 41 (Firefox 41 / Thunderbird 41 / SeaMonkey 2.38) and to comply with the ES6 specification, parenthesized destructuring patterns, like ([a, b]) = [1, 2] or ({a, b}) = { a: 1, b: 2 } , are now considered invalid and will throw a SyntaxError . See Jeff Walden's blog post and bug 1146136 for more details.
• Assignment operators
• "ES6 in Depth: Destructuring" on hacks.mozilla.org

© 2016 Mozilla Contributors Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-us/docs/web/javascript/reference/operators/destructuring_assignment

Anjan Dutta

Extract data from arrays and objects in javascript.

Updated on: 2024-01-16

In this blog, we are going to explore how to extract data from arrays and objects in javaScript using destructuring.

The JavaScript destructuring assignment is a notable feature introduced in ES6. It simplifies the process of writing clear and concise JavaScript assignment expressions, allowing us to extract values from arrays or properties from objects.

How to extract data from array in javascript:

Let me describe the above code snippet step by step:

var ten, twenty; : Declares two variables, ten and twenty, without initializing them.

[ten, twenty] = [10, 20]; : Utilizes destructuring assignment to assign values from an array to the variables ten and twenty. The values on the right side of the equal sign ( [10, 20] ) are an array with two elements, and the destructuring assignment syntax [ten, twenty] extracts and assigns these values to the corresponding variables.

Before extracting the values from an array, it's possible to both declare and assign a variable in a single line, as demonstrated below:

The above code snippet initializes two variables, ten and twenty , using destructuring assignment from an array [10, 20] . It then prints the value of both the variables to the console.

How to extract value from array in javascript

How to extract value from object in javascript:.

Another way of getting data is shown below.

How to extract data from object in javascript

Javascript destructuring vs constructing data:.

Javascript has the ability to construct multiple data at a time.

For example, const response = {status: 200, message: "OK"};

Here, multiple data getting assigned in a single declaration.

Similarly, there was no direct feature in javascript for deconstructing or extracting multiple data at a time until ES6 introduced destructuring.

Where can we use ES6 Destructuring?

A smart implementation of destructuring can resolve a wide range of problems. Problems varying from variable declarations to swapping values or even iterating through complex objects in a loop, all can be handled using ES6 destructuring.

Variable declarations using destructuring

Variable value assignment using destructuring, passing of function parameter using destructuring.

Here, in the first function call, x and y will get values 7 and 3 respectively. But, in the second function call it will return zero because, if you look closely, we have passed default parameters [0, 0] in the function declaration. This is a good way of handling an exception.

Destructuring of a for-of iteration

We can also use destructuring with for-of loops like below example. This approach helps us in getting the current array index as well as distinct value for the same index.

And here is another interesting method.

How to get data from array of objects in javascript

The below example shows how to extract object from an array and then iterate through a complex objects' value without using an index.

how to extract array from object in javascript

How to swap variable values using destructuring.

Using destructuring, swapping of variable value is really easy. Below example exactly shows how it is done.

Extract data from arrays and objects in javascript

Destructuring involves a source and a target. The source is always the data to be destructured or right-hand side of an assignment. Whereas target is a pattern that is to be used on the left-hand side of an assignment.

A target can be any of the three following patterns.

Using Default values in destructuring

Default values can be assigned to destructuring target variables. The advantage of it is, if an extracted assignment has an undefined value then the default value will be assigned to the target. This is a good practice of handling exception.

The syntax is as follows.

Here, a default value 0 will be assigned in case if no corresponding value extracted from the destructuring assignment.

Elision or ignoring returned values in destructuring

In some cases, there might be a requirement of ignoring certain value(s) during destructuring of an assignment.

It can be done using the following syntax.

How to extract remaining data from object using the javascript Rest operator (...)

Rest operator lets us assign the rest of an iterable to a target variable.

Destructuring prevents code redundancy and can be applied to any iterable on the right side of an assignment operator. While it is compatible with various iterators such as set, map, arrays, and objects, we will focus on array and object destructuring, which are the most commonly used patterns.

In conclusion, these examples highlight the flexibility and readability that destructuring provides in JavaScript. Whether dealing with arrays or objects, destructuring allows for concise and expressive code, enhancing the clarity and efficiency of variable assignments.

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java:S1121: "Extract the assignment out of this expression."

I've a big java project given by my teacher for the exam, and analysis with SonarCloud gives me this Code Smell to fix on this function

In particular it's about if(isBidAction) . I thought to put isBidAction=true immediately upon that if to fix this. Could it be a solution?

• if-statement

• That's the same thing... I suggest you to use short-return... I don't see any errors... –  DynoZ Commented Mar 29, 2021 at 6:07
• return isBidAction ? takerReserveBidPrice : reserveBidPrice ? –  Johannes Kuhn Commented Mar 29, 2021 at 6:08
• Did SonarQube give you any error message more specific than this? I don't see any code smell actually. –  Tim Biegeleisen Commented Mar 29, 2021 at 6:08
• 2 Seems like you used if(isBidAction = true) instead of if(isBidAction == true) . In any case, if(isBidAction) is what you want to use, if you should stick to the if statement at all (you can change this to a conditional expression) –  ernest_k Commented Mar 29, 2021 at 6:09
• 2 When you look at the Sonar Qube Rule Repository RSPEC-1121 you find that the non-compliant code contains an assignment. So your code should be OK - maybe the complaint is about an older version that was if(isBidAction = true) {...} –  Thomas Kläger Commented Mar 29, 2021 at 6:16

SonarQube tells you that methods whose the whole logic executions depend on an input boolean parameter is a bad practice. Your structure is equivalent to:

The usage is as follows:

This method doesn't bring any value. It only hides the if-else statement which could may decrease the code readability. You should inline it:

In your case:

When the logic is more complex, you may create a method to extract required value, but still don't hide the if-else condition:

Of course there is no sense to create a such method only to execute the makerOrder.reserveBidPrice condition (it is too simple).

Btw. SonarQube marks only these methods which full logic executions depend on a simple condition. It means that this

would be marked as issue, but this code not:

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