Line Value Assignment. Assignment (computer science)

Line Value Assignment

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

var a, b, rest; [a, b] = [10, 20]; console.log(a); // 10 console.log(b); // 20 [a, b, ...rest] = [10, 20, 30, 40, 50]; console.log(a); // 10 console.log(b); // 20 console.log(rest); // [30, 40, 50] ({ a, b } = { a: 10, b: 20 }); console.log(a); // 10 console.log(b); // 20 // Stage 3 proposal ({a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40}); console.log(a); // 10 console.log(b); // 20 console.log(rest); //{c: 30, d: 40}

Description

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

var x = [1, 2, 3, 4, 5];

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

var x = [1, 2, 3, 4, 5]; var [y, z] = x; console.log(y); // 1 console.log(z); // 2

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

Array destructuring

Basic variable assignment

var foo = ['one', 'two', 'three']; var [one, two, three] = foo; console.log(one); // "one" console.log(two); // "two" console.log(three); // "three"

Assignment separate from declaration

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

var a, b; [a, b] = [1, 2]; console.log(a); // 1 console.log(b); // 2

Default values

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

var a, b; [a=5, b=7] = [1]; console.log(a); // 1 console.log(b); // 7

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).

var a = 1; var b = 3; [a, b] = [b, a]; console.log(a); // 3 console.log(b); // 1

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, returns the values as its output, which can be parsed in a single line with destructuring.

function f() { return [1, 2]; } var a, b; [a, b] = f(); console.log(a); // 1 console.log(b); // 2

Ignoring some returned values

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

function f() { return [1, 2, 3]; } var [a, , b] = f(); console.log(a); // 1 console.log(b); // 3

You can also ignore all returned values:

[,,] = f();

Assigning the rest of an array to a variable

When destructuring an array, you can unpack and assign the remaining part of it to a variable using the rest pattern:

var [a, ...b] = [1, 2, 3]; console.log(a); // 1 console.log(b); // [2, 3]

Note that a will be thrown if a trailing comma is used on the left-hand side with a rest element:

var [a, ...b,] = [1, 2, 3]; // SyntaxError: rest element may not have a trailing comma

Unpacking values from a regular expression match

When the regular expression 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 unpack the parts out of this array easily, ignoring the full match if it is not needed.

function parseProtocol(url) { var parsedURL = /^(\w+)\:\/\/([^\/]+)\/(.*)$/.exec(url); if (!parsedURL) { return false; } console.log(parsedURL); // ["https://developer.mozilla.org/en-US/Web/JavaScript", "https", "developer.mozilla.org", "en-US/Web/JavaScript"] var [, protocol, fullhost, fullpath] = parsedURL; return protocol; } console.log(parseProtocol('https://developer.mozilla.org/en-US/Web/JavaScript')); // "https"

Object destructuring

Basic assignment

var o = {p: 42, q: true}; var {p, q} = o; console.log(p); // 42 console.log(q); // true

Assignment without declaration

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

var a, b; ({a, b} = {a: 1, b: 2});

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

is not valid stand-alone syntax, as the on the left-hand side is considered a block and not an object literal.

However, is valid, as is

Assigning to new variable names

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

var o = {p: 42, q: true}; var {p: foo, q: bar} = o; console.log(foo); // 42 console.log(bar); // true

Default values

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

var {a = 10, b = 5} = {a: 3}; console.log(a); // 3 console.log(b); // 5

Assigning to new variables names and providing default values

A property can be both 1) unpacked from an object and assigned to a variable with a different name and 2) assigned a default value in case the unpacked value is .

var {a:aa = 10, b:bb = 5} = {a: 3}; console.log(aa); // 3 console.log(bb); // 5

Setting a function parameter's default value

ES5 version

ES2015 version

function drawES2015Chart({size = 'big', cords = {x: 0, y: 0}, radius = 25} = {}) { console.log(size, cords, radius); // do some chart drawing } drawES2015Chart({ cords: {x: 18, y: 30}, radius: 30 });

In the function signature for above, the destructured left-hand side is assigned to an empty object literal on the right-hand side: . You could have also written the function without the right-hand side assignment. However, if you leave out the right-hand side assignment, the function will look for at least one argument to be supplied when invoked, whereas in its current form, you can simply call without supplying any parameters. The current design is useful if you want to be able to call the function without supplying any parameters, the other can be useful when you want to ensure an object is passed to the function.

Nested object and array destructuring

For of iteration and destructuring

var people = [ { name: 'Mike Smith', family: { mother: 'Jane Smith', father: 'Harry Smith', sister: 'Samantha Smith' }, age: 35 }, { name: 'Tom Jones', family: { mother: 'Norah Jones', father: 'Richard Jones', brother: 'Howard Jones' }, age: 25 } ]; for (var {name: n, family: {father: f}} of people) { console.log('Name: ' + n + ', Father: ' + f); } // "Name: Mike Smith, Father: Harry Smith" // "Name: Tom Jones, Father: Richard Jones"

Unpacking fields from objects passed as function parameter

This unpacks the , and 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.

let key = 'z'; let {[key]: foo} = {z: 'bar'}; console.log(foo); // "bar"

Rest in Object Destructuring

The Rest/Spread Properties for ECMAScript proposal (stage 3) adds the rest syntax to destructuring. Rest properties collect the remaining own enumerable property keys that are not already picked off by the destructuring pattern.

let {a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40} a; // 10 b; // 20 rest; // { c: 30, d: 40 }

Invalid JavaScript identifier as a property name

Destructuring can be used with property names that are not valid JavaScript identifiers by providing an alternative identifer that is valid.

const foo = { 'fizz-buzz': true }; const { 'fizz-buzz': fizzBuzz } = foo; console.log(fizzBuzz); // "true"

Specifications

Browser compatibility

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic support	49	14	41¹	No	Yes	8
Computed property names	49	14	34	No	Yes	No
Rest in arrays	49	14²	34	No	Yes	No
Rest in objects	60	No	55	No	Yes	No

Feature	Android webview	Chrome for Android	Edge mobile	Firefox for Android	Opera Android	iOS Safari	Samsung Internet
Basic support	49	49	14	41¹	Yes	8	?
Computed property names	49	49	14	34	Yes	No	?
Rest in arrays	49	49	14²	34	Yes	No	?
Rest in objects	60	60	No	55	Yes	No	?

1. Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.

2. From version 14: this feature is behind the preference.

Desktop	Mobile	Server
Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Edge Mobile	Firefox for Android	Opera for Android	iOS Safari	Samsung Internet	Node.js
Basic support	Full support 49	Full support 14	Full support 41 Notes Full support 41 Notes Notes Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.	No support No	Full support Yes	Full support 8	Full support 49	Full support 49	Full support 14	Full support 41 Notes Full support 41 Notes Notes Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.	Full support Yes	Full support 8	?	Full support Yes
Computed property names	Full support 49	Full support 14	Full support 34	No support No	Full support Yes	No support No	Full support 49	Full support 49	Full support 14	Full support 34	Full support Yes	No support No	?	Full support Yes
Rest in arrays	Full support 49	Full support 14 Disabled Full support 14 Disabled Disabled From version 14: this feature is behind the preference.	Full support 34	No support No	Full support Yes	No support No	Full support 49	Full support 49	Full support 14 Disabled Full support 14 Disabled Disabled From version 14: this feature is behind the preference.	Full support 34	Full support Yes	No support No	?	Full support Yes
Rest in objects Experimental	Full support 60	No support No	Full support 55	No support No	Full support Yes	No support No	Full support 60	Full support 60	No support No	Full support 55	Full support Yes	No support No	?	Full support Yes

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
Experimental. Expect behavior to change in the future.: Experimental. Expect behavior to change in the future.
See implementation notes.: See implementation notes.
User must explicitly enable this feature.: User must explicitly enable this feature.

Tuple Assignment with unpacking¶

Python has a very powerful tuple assignment feature that allows a tuple of variable names on the left of an assignment statement to be assigned values from a tuple on the right of the assignment. Another way to think of this is that the tuple of values is unpacked into the variable names.

This does the equivalent of seven assignment statements, all on one easy line. One requirement is that the number of variables on the left must match the number of elements in the tuple.

Once in a while, it is useful to swap the values of two variables. With conventional assignment statements, we have to use a temporary variable. For example, to swap and :

Tuple assignment solves this problem neatly:

The left side is a tuple of variables; the right side is a tuple of values. Each value is assigned to its respective variable. All the expressions on the right side are evaluated before any of the assignments. This feature makes tuple assignment quite versatile.

Naturally, the number of variables on the left and the number of values on the right have to be the same.

Python even provides a way to pass a single tuple to a function and have it be unpacked for assignment to the named parameters.

(cp09_tuple4)

If you run this, you will be get an error caused by line 7, where it says that the function add is expecting two parameters, but you’re only passing one parameter (a tuple). In line 6 you’ll see that the tuple is unpacked and 5 is bound to x, 4 to y.

Don’t worry about mastering this idea yet. But later in the course, if you come across some code that someone else has written that uses the * notation inside a parameter list, come back and look at this again.

keyword-params-91: If you want a function to return two values, contained in variables x and y, which of the following methods will work?

keyword-params-92: Consider the following alternative way to swap the values of variables x and y. What’s wrong with it?

# assume x and y already have values assigned to themy=xx=y

(name,surname,birth_year,movie,movie_year,profession,birth_place)=julia

(a,b,c,d)=(1,2,3)# ValueError: need more than 3 values to unpack

Next Section - Unpacking Dictionary Items