Symbol.isConcatSpreadable

## Value

The well-known symbol `Symbol.isConcatSpreadable`.

{{js_property_attributes(0, 0, 0)}}

## Description

The `[Symbol.isConcatSpreadable]` property 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, {{jsxref("Array.prototype.concat()")}} spreads (flattens) arrays into its result:

```js
const alpha = ["a", "b", "c"];
const numeric = [1, 2, 3];

const alphaNumeric = alpha.concat(numeric);

console.log(alphaNumeric); // Result: ['a', 'b', 'c', 1, 2, 3]

const alpha = ["a", "b", "c"];
const numeric = [1, 2, 3];

numeric[Symbol.isConcatSpreadable] = false;
const alphaNumeric = alpha.concat(numeric);

console.log(alphaNumeric); // Result: ['a', 'b', 'c', [1, 2, 3] ]

const x = [1, 2, 3];

const fakeArray = {
  [Symbol.isConcatSpreadable]: true,
  length: 2,
  0: "hello",
  1: "world",
};

x.concat(fakeArray); // [1, 2, 3, "hello", "world"]