title | shortTitle | type | language | tags | cover | excerpt | dateModified | ||
---|---|---|---|---|---|---|---|---|---|
Use JavaScript for loops if you need to break out early |
For loops for early breaking |
tip |
javascript |
|
armchair |
Iteration in JavaScript can be done a handfuld of ways, most often using array methods, but sometimes a `for` loop is the best option. |
2021-06-12 |
The usefulness of the humble for
loop in modern JavaScript is rarely talked about. Apart from it being particularly useful in asynchronous operation scenarios, it can also make your code a lot more performant shall you need to break out of a loop early. Consider the following example:
const smallArray = [0, 2];
const largeArray = Array.from({ length: 1000 }, (_, i) => i);
const areEqual = (a, b) => {
let result = true;
a.forEach((x, i) => {
if (!result) return;
if (b[i] === undefined || x !== b[i]) result = false;
});
return result;
}
areEqual(largeArray, smallArray); // false
// Will loop over all items in `largeArray`
Obviously, the code isn't optimized, but it highlights the issue of array methods, such as Array.prototype.forEach()
being unable to break out of the loop early. To counteract this, we could use a for
loop and an early return
instead:
const smallArray = [0, 2];
const largeArray = Array.from({ length: 1000 }, (_, i) => i);
const areEqual = (a, b) => {
for (let i in a) {
if (b[i] === undefined || a[i] !== b[i]) return false;
}
return true;
}
areEqual(largeArray, smallArray); // false
// Will only loop until the first mismatch is encountered