The well-known 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:

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]

When setting Symbol.isConcatSpreadable to false, you can disable the default behavior:

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] ]

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:

const x = [1, 2, 3];

const 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()