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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:

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

var 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:

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

numeric[Symbol.isConcatSpreadable] = false;
var 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:

var x = [1, 2, 3];

var 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.


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Symbol.isconcatspreadable' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Symbol.isconcatspreadable' in that specification.
Draft No change.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
isConcatSpreadableChrome Full support 48Edge Full support 15Firefox Full support 48IE No support NoOpera Full support YesSafari Full support YesWebView Android Full support 48Chrome Android Full support 48Edge Mobile Full support YesFirefox Android Full support 48Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support 6.0.0


Full support  
Full support
No support  
No support

See also

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