Iterator.prototype.windows()
The windows() method of Iterator instances returns a new iterator helper object that yields overlapping element sequences as arrays. A sliding window sweeps across the input iterator; each time, a new element is added to the right while the leftmost element is removed.
For yielding non-overlapping sequences (i.e., chunking), see Iterator.prototype.chunks().
Syntax
windows(windowSize)
windows(windowSize, underSized)
Parameters
windowSize-
The number of elements in the sliding window. Must be a positive integer less than 232 (the maximum array length).
underSizedOptional-
A string specifying what to do when the number of elements in the iterator is less than
windowSizebut greater than 0. It can be one of the following:"only-full"(default)-
Ignore the partial window. The returned iterator immediately completes as if the input is empty.
"allow-partial"-
Yield all elements in the partial window as an array (with length less than
windowSize).
Note that when the input is empty, the returned iterator always immediately completes without yielding an empty array.
Return value
A new iterator helper object. The first time the returned iterator helper's next() method is called, the current iterator is immediately advanced by windowSize elements. If that many elements are available, they are yielded together as an array. Otherwise, the behavior depends on the underSized argument: the iterator either immediately completes without yielding anything (with "only-full" or with a fully empty input), or an array of length less than windowSize is yielded.
Afterwards, each time, the yielded array consists of the previous array, the leftmost element removed, a new element retrieved from the input iterable appended. When the input iterable is exhausted, the iterator helper also completes.
Exceptions
TypeError-
Thrown if
limitis not an integer, or ifunderSizedis not one of the allowed values. RangeError-
Thrown if
limitis less than 1 or greater than 232－1.
Examples
Moving average
The following example creates an iterator that yields terms in the Fibonacci sequence. Then, a new iterator is created, each time yielding the average of two adjacent elements.
function* fibonacci() {
let current = 1;
let next = 1;
while (true) {
yield current;
[current, next] = [next, current + next];
}
}
const averaged = fibonacci()
.windows(2)
.map((window) => (window[0] + window[1]) / 2);
console.log(averaged.next().value); // 1 (1, 1)
console.log(averaged.next().value); // 1.5 (1, 2)
console.log(averaged.next().value); // 2.5 (2, 3)
console.log(averaged.next().value); // 4 (3, 5)
This process, known as a moving average, is very useful in signal processing.
Enumerating substrings
The following example enumerates all substrings of a given length in a string. The windows() method is used to create a sliding window of characters, which are then joined together to form substrings.
Note:
This is probably much less efficient than manually iterating a number index and calling the String.prototype.substring() method.
const chars = Iterator.from("MASSACHUSETTS");
const substrings = chars.windows(3).map((window) => window.join(""));
for (const substring of substrings) {
console.log(substring);
}
// MAS
// ASS
// SSA
// ...
Specifications
| Specification |
|---|
| Iterator Chunking # sec-iterator.prototype.windows |