Iterator.prototype.map()
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
The map()
method of Iterator
instances returns a new iterator helper that yields elements of the iterator, each transformed by a mapping function.
Syntax
map(callbackFn)
Parameters
callbackFn
-
A function to execute for each element produced by the iterator. Its return value is yielded by the iterator helper. The function is called with the following arguments:
Return value
A new iterator helper. Each time the iterator helper's next()
method is called, it gets the next element from the underlying iterator, applies callbackFn
, and yields the return value. When the underlying iterator is completed, the iterator helper is also completed (the next()
method produces { value: undefined, done: true }
).
Description
The main advantage of iterator helpers over array methods is their ability to work with infinite iterators. With infinite iterators, map()
allows you to create a new iterator that, when iterated, produces transformed elements.
Examples
Using map()
The following example creates an iterator that yields terms in the Fibonacci sequence, transforms it into a new sequence with each term squared, and then reads the first few terms:
function* fibonacci() {
let current = 1;
let next = 1;
while (true) {
yield current;
[current, next] = [next, current + next];
}
}
const seq = fibonacci().map((x) => x ** 2);
console.log(seq.next().value); // 1
console.log(seq.next().value); // 1
console.log(seq.next().value); // 4
Using map() with a for...of loop
map()
is most convenient when you are not hand-rolling the iterator. Because iterators are also iterable, you can iterate the returned helper with a for...of
loop:
for (const n of fibonacci().map((x) => x ** 2)) {
console.log(n);
if (n > 30) {
break;
}
}
// Logs:
// 1
// 1
// 4
// 9
// 25
// 64
This is equivalent to:
for (const n of fibonacci()) {
const n2 = n ** 2;
console.log(n2);
if (n2 > 30) {
break;
}
}
Specifications
Specification |
---|
Iterator Helpers # sec-iteratorprototype.map |
Browser compatibility
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