Symbol.isConcatSpreadable

The Symbol.isConcatSpreadable static data property represents the well-known symbol Symbol.isConcatSpreadable. The Array.prototype.concat() method looks up this symbol on each object being concatenated to determine if it should be treated as an array-like object and flattened to its array elements.

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.