Web/JavaScript/Reference/Global objects/Symbol/isConcatSpreadable

The Symbol.isConcatSpreadable well-known symbol is used to configure if an object should be flattened to its array elements when using the Array.prototype.concat() method.

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Description

The @@isConcatSpreadable symbol (Symbol.isConcatSpreadable) can be defined as an own or inherited property and its value is a boolean. It can control behavior for arrays and array-like objects:

For array objects, the default behavior is to spread (flatten) elements. Symbol.isConcatSpreadable can avoid flattening in these cases.
For array-like objects, the default behavior is no spreading or flattening. Symbol.isConcatSpreadable can force flattening in these cases.

Property attributes of `Symbol.isConcatSpreadable`
Writable	no
Enumerable	no
Configurable	no

Examples

Arrays

By default, Array.prototype.concat() spreads (flattens) arrays into its result:

let alpha = ['a', 'b', 'c'], 
let numeric = [1, 2, 3]

let alphaNumeric = alpha.concat(numeric)

console.log(alphaNumeric)  // Result: ['a', 'b', 'c', 1, 2, 3]

When setting Symbol.isConcatSpreadable to false, you can disable the default behavior:

let alpha = ['a', 'b', 'c'], 
let numeric = [1, 2, 3]

numeric[Symbol.isConcatSpreadable] = false
let alphaNumeric = alpha.concat(numeric)

console.log(alphaNumeric)  // Result: ['a', 'b', 'c', [1, 2, 3] ]

Array-like objects

For array-like objects, the default is to not spread. Symbol.isConcatSpreadable needs to be set to true in order to get a flattened array:

let x = [1, 2, 3]

let fakeArray = { 
  [Symbol.isConcatSpreadable]: true, 
  length: 2, 
  0: 'hello', 
  1: 'world' 
}

x.concat(fakeArray)  // [1, 2, 3, "hello", "world"]

Note: The length property is used to control the number of object properties to be added. In the above example, length:2 indicates two properties has to be added.

Specifications

Specification

ECMAScript (ECMA-262)The definition of 'Symbol.isconcatspreadable' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`isConcatSpreadable`	Chrome Full support 48	Edge Full support 15	Firefox Full support 48	IE No support No	Opera Full support 35	Safari Full support 10	WebView Android Full support 48	Chrome Android Full support 48	Firefox Android Full support 48	Opera Android Full support 35	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0

Legend

Full support: Full support
No support: No support