---
hide_title: false
hide_table_of_contents: false
pagination_next: null
pagination_prev: null
---
# 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()`](../../globals/Array/prototype/concat.mdx) method.

## Value

The well-known symbol `Symbol.isConcatSpreadable`.

## Description

The `Symbol.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.
