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.


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
isConcatSpreadable 48 15 48 No 35 10
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
isConcatSpreadable 48 48 48 35 10 5.0
isConcatSpreadable 6.0.0

See also

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