Symbol.isConcatSpreadable

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.

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] ]

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"]

• Polyfill of Symbol.isConcatSpreadable in core-js
• Array.prototype.concat()