Iterator.prototype.every()
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
The every()
method of Iterator
instances is similar to Array.prototype.every()
: it tests whether all elements produced by the iterator pass the test implemented by the provided function. It returns a boolean value.
Syntax
every(callbackFn)
Parameters
callbackFn
-
A function to execute for each element produced by the iterator. It should return a truthy value to indicate the element passes the test, and a falsy value otherwise. The function is called with the following arguments:
Return value
true
if callbackFn
returns a truthy value for every element. Otherwise, false
.
Description
every()
iterates the iterator and invokes the callbackFn
function once for each element. It returns false
immediately if the callback function returns a falsy value. Otherwise, it iterates until the end of the iterator and returns true
. If every()
returns false
, the underlying iterator is closed by calling its return()
method.
The main advantage of iterator helpers over array methods is that they are lazy, meaning that they only produce the next value when requested. This avoids unnecessary computation and also allows them to be used with infinite iterators. With infinite iterators, every()
returns false
as soon as the first falsy value is found. If the callbackFn
always returns a truthy value, the method never returns.
Examples
Using every()
function* fibonacci() {
let current = 1;
let next = 1;
while (true) {
yield current;
[current, next] = [next, current + next];
}
}
const isEven = (x) => x % 2 === 0;
console.log(fibonacci().every(isEven)); // false
const isPositive = (x) => x > 0;
console.log(fibonacci().take(10).every(isPositive)); // true
console.log(fibonacci().every(isPositive)); // Never completes
Calling every()
always closes the underlying iterator, even if the method early-returns. The iterator is never left in a half-way state.
const seq = fibonacci();
console.log(seq.every(isEven)); // false
console.log(seq.next()); // { value: undefined, done: true }
Specifications
Specification |
---|
Iterator Helpers # sec-iteratorprototype.every |
Browser compatibility
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