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.

Examples

Arrays

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"]

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Symbol.isconcatspreadable' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of 'Symbol.isconcatspreadable' in that specification.	Draft	No change.

Browser compatibility

See also

• Array.prototype.concat()