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.


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



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.


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

Browser compatibility

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ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
isConcatSpreadableChrome Full support 48Edge Full support 15Firefox Full support 48IE No support NoOpera Full support 35Safari Full support 10WebView Android Full support 48Chrome Android Full support 48Firefox Android Full support 48Opera Android Full support 35Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0


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See also