Baseline 2025
Newly available
The drop() method of Iterator instances returns a new iterator helper object that skips the given number of elements at the start of this iterator.
limit
The number of elements to drop from the start of the iteration.
A new iterator helper object. The first time the returned iterator helper's next() method is called, the current iterator is immediately advanced by limit elements, and then the next element (the limit+1-th element) is yielded. The iterator helper then yields the remaining elements one-by-one. If the current iterator has fewer than limit elements, the new iterator helper will be immediately completed the first time next() is called.
RangeError
Thrown if limit becomes NaN or negative when converted to an integer.
The following example creates an iterator that yields terms in the Fibonacci sequence, starting from the 3rd term by dropping the first two terms:
function* fibonacci() {
let current = 1;
let next = 1;
while (true) {
yield current;
[current, next] = [next, current + next];
}
}
const seq = fibonacci().drop(2);
console.log(seq.next().value); // 2
console.log(seq.next().value); // 3
This is equivalent to:
const seq = fibonacci();
seq.next();
seq.next();
drop() 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().drop(2)) {
console.log(n);
if (n > 30) {
break;
}
}
// Logs:
// 2
// 3
// 5
// 8
// 13
// 21
// 34
You can combine drop() with Iterator.prototype.take() to get a slice of an iterator:
for (const n of fibonacci().drop(2).take(5)) {
// Drops the first two elements, then takes the next five
console.log(n);
}
// Logs:
// 2
// 3
// 5
// 8
// 13
for (const n of fibonacci().take(5).drop(2)) {
// Takes the first five elements, then drops the first two
console.log(n);
}
// Logs:
// 2
// 3
// 5
When the limit is negative or NaN, a RangeError is thrown:
fibonacci().drop(-1); // RangeError: -1 must be positive
fibonacci().drop(undefined); // RangeError: undefined must be positive
When the limit is larger than the total number of elements the iterator can produce (such as Infinity), the returned iterator helper will instantly drop all elements and then be completed the first time next() is called. If the current iterator is infinite, the returned iterator helper will never complete.
fibonacci().drop(Infinity).next(); // Never ends
new Set([1, 2, 3]).values().drop(Infinity).next(); // { value: undefined, done: true }
new Set([1, 2, 3]).values().drop(4).next(); // { value: undefined, done: true }
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