Array comprehensions

The array comprehension syntax is a JavaScript expression which allows you to quickly assemble a new array based on an existing one. Comprehensions exist in many programming languages.

See below for differences to the old array comprehension syntax in SpiderMonkey, based on proposals for ECMAScript 4.

Syntax

[for (x of iterable) x]
[for (x of iterable) if (condition) x]
[for (x of iterable) for (y of iterable) x + y]

Description

Inside array comprehensions, these two kinds of components are allowed:

• for...of and
• if

The for-of iteration is always the first component. Multiple for-of iterations or if statements are allowed.

Array comprehension was previously proposed to be standardized in ECMAScript 7, it provide a useful shortcut for constructing a new array based on the contents of another. Comprehensions can often be used in place of calls to map() and filter(), or as a way of combining the two.

The following comprehension takes an array of numbers and creates a new array of the double of each of those numbers.

var numbers = [1, 2, 3, 4];
var doubled = [for (i of numbers) i * 2];
console.log(doubled); // logs 2,4,6,8

This is equivalent to the following map() operation:

var doubled = numbers.map(i => i * 2);

Comprehensions can also be used to select items that match a particular expression. Here is a comprehension which selects only even numbers:

var numbers = [1, 2, 3, 21, 22, 30];
var evens = [for (i of numbers) if (i % 2 === 0) i];
console.log(evens); // logs 2,22,30

filter() can be used for the same purpose:

var evens = numbers.filter(i => i % 2 === 0);

map() and filter() style operations can be combined into a single array comprehension. Here is one that filters just the even numbers, then creates an array containing their doubles:

var numbers = [1, 2, 3, 21, 22, 30];
var doubledEvens = [for (i of numbers) if (i % 2 === 0) i * 2];
console.log(doubledEvens); // logs 4,44,60

The square brackets of an array comprehension introduce an implicit block for scoping purposes. New variables (such as i in the example) are treated as if they had been declared using let. This means that they will not be available outside of the comprehension.

The input to an array comprehension does not itself need to be an array; iterators and generators can also be used.

Even strings may be used as input; to achieve the filter and map actions (under Array-like objects) above:

var str = 'abcdef';
var consonantsOnlyStr = [for (c of str) if (!(/[aeiouAEIOU]/).test(c)) c].join(''); // 'bcdf'
var interpolatedZeros = [for (c of str) c+'0' ].join(''); // 'a0b0c0d0e0f0'

Again, the input form is not preserved, so we have to use join() to revert back to a string.

Examples

Simple array comprehensions

[for (i of [ 1, 2, 3 ]) i*i ]; 
// [ 1, 4, 9 ]

var abc = [ "A", "B", "C" ];
[for (letters of abc) letters.toLowerCase()];
// [ "a", "b", "c" ]

Array comprehensions with if statement

var years = [ 1954, 1974, 1990, 2006, 2010, 2014 ];
[for (year of years) if (year > 2000) year];
// [ 2006, 2010, 2014 ]
[for (year of years) if (year > 2000) if(year < 2010) year];
// [ 2006], the same as below:
[for (year of years) if (year > 2000 && year < 2010) year];
// [ 2006] 

Array comprehensions compared to map and filter

An easy way to understand array comprehension syntax, is to compare it with the Array map and filter methods:

var numbers = [ 1, 2, 3 ];

numbers.map(function (i) { return i * i });
numbers.map(i => i*i);
[for (i of numbers) i*i ];
// all are [ 1, 4, 9 ]

numbers.filter(function (i) { return i < 3 });
numbers.filter(i => i < 3);
[for (i of numbers) if (i < 3) i];
// all are [ 1, 2 ]

Array comprehensions with two arrays

Using two for-of iterations to work with two arrays:

var numbers = [ 1, 2, 3 ];
var letters = [ "a", "b", "c" ];

var cross = [for (i of numbers) for (j of letters) i+j];
// [ "1a", "1b", "1c", "2a", "2b", "2c", "3a", "3b", "3c" ]

var grid = [for (i of numbers) [for (j of letters) i+j]];
// [
//  ["1a", "1b", "1c"],
//  ["2a", "2b", "2c"],
//  ["3a", "3b", "3c"]
// ]

[for (i of numbers) if (i > 1) for (j of letters) if(j > "a") i+j]
// ["2b", "2c", "3b", "3c"], the same as below:

[for (i of numbers) for (j of letters) if (i > 1) if(j > "a") i+j]
// ["2b", "2c", "3b", "3c"]

[for (i of numbers) if (i > 1) [for (j of letters) if(j > "a") i+j]]
// [["2b", "2c"], ["3b", "3c"]], not the same as below:

[for (i of numbers) [for (j of letters) if (i > 1) if(j > "a") i+j]]
// [[], ["2b", "2c"], ["3b", "3c"]]

Specifications

Was initially in the ECMAScript 6 draft, but got removed in revision 27 (August 2014). Please see older revisions of ES 6 for specification semantics.

Browser compatibility

SpiderMonkey-specific implementation notes

• let as an identifier is not supported as let is currently only available to JS version 1.7 and XUL scripts tags.
• Destructuring in comprehensions is not supported yet (bug 980828).

Differences to the older JS1.7/JS1.8 comprehensions

Old comprehensions syntax (do not use anymore!):

[X for (Y in Z)]
[X for each (Y in Z)]
[X for (Y of Z)]

Differences:

• ES7 comprehensions create one scope per "for" node instead of the comprehension as a whole.
  • Old: [()=>x for (x of [0, 1, 2])][1]() // 2
  • New: [for (x of [0, 1, 2]) ()=>x][1]() // 1, each iteration creates a fresh binding for x.
• ES7 comprehensions start with "for" instead of the assignment expression.
  • Old: [i * 2 for (i of numbers)]
  • New: [for (i of numbers) i * 2]
• ES7 comprehensions can have multiple if and for components.
• ES7 comprehensions only work with for...of and not with for...in iterations.

See Bug 1220564, comment 42 for suggestions on updating code.

See also

• for...of
• Generator comprehensions