ITERATION AND SPREAD OPERATORS

ECMAScript 6 introduced iterators and the spread operator, which are especially useful in the context of collection reference types. These new tools allow for easy interoperability, cloning, and modification of collection types.

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NOTE   The Iterators and Generators chapter offers more coverage on exactly how iterators work.

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As shown earlier in the chapter, four native collection reference types define a default iterator:

• Array
• All typed arrays
• Map
• Set

Trivially, this means that all support ordered iteration and can be passed to a for..of loop:

let iterableThings = [
 Array.of(1, 2),
 typedArr = Int16Array.of(3, 4),
 new Map([[5, 6], [7, 8]]),
 new Set([9, 10])
];
  

for (const iterableThing of iterableThings) {
 for (const x of iterableThing) {
  console.log(x);
 }
}

// 1
// 2
// 3
// 4
// [5, 6]
// [7, 8]
// 9
// 10

This also means that all these types are compatible with the spread operator. The spread operator is especially...