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.

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

BCD tables only load in the browser

See also