`Symbol.isConcatSpreadable` can only work on Array object?

[OahMada]

Yes, I promise I've read the Contributions Guidelines (please feel free to remove this line).

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Please type "I already searched for this issue": I already searched for this issue Edition: (1st or 2nd) 1st Book Title: ES6 & Beyond Chapter: 7, Meta Programming Section Title: Well-Known Symbols Problem: In the Symbol.isConcatSpreadable part, you said "The @@isConcatSpreadable symbol can be defined as a boolean property on any object (like an array or other iterable) ," but I don't think any object/iterable can fit here, since concat is a method on Array or String prototype objects. I tried with a custom non-array one, it didn't work.

[getify]

Many details from 1st edition are no longer accurate. However, as the contribution guidelines spell out, I'm not accepting any updates to first edition books. Thanks.

[OahMada]

sorry

[mohitpal2621]

It can be invoked like this:

const arrayLike = {
  [Symbol.isConcatSpreadable]: true,
  length: 2,
  0: 1,
  1: 2,
  2: 99, // ignored by concat() since length is 2
};
console.log(Array.prototype.concat.call(arrayLike, 3, 4)); // [1, 2, 3, 4]