js 如何让forEach可以break？

While working with Java Script, all of us must have surely run into the case where we need to loop through an array and break the running loop if a certain condition is met. There are many methods how we can loop through an array but we are always looking for the most efficient way to do that.

Our favorite loop method: Array.prototype.forEach

forEach is such an useful method.
But…how can I break the forEach loop?
Well… you can’t break forEach.

var arr = ["Kathmandu", "Pokhara", "Lumbini", "Gorkha"];
 
arr.forEach(function(value, index, _arr) {
console.log(index + ": " + value);
return false;
});

The result should look like

Kathmandu
Pokhara
Lumbini
Gorkha

It’s still iterating through all items in the array. What should I do if I want to stop the loop based on the condition? We either set a variable that changes only if the condition is met, but it will still mean that the loop will iterate until the array is exhausted to the last item.

So, what do we do?? It’s simple.

Don’t use “forEach”. Use “some” or “every”.

Array.prototype.some

Array.prototype.some is pretty much the same as forEach but it break when the callback returns true.

Example:

var arr= ["Kathmandu", "Pokhara", "Lumbini", "Gorkha"];
 
arr.some(function (value, index, _arr) {
console.log(index + ": " + value);
return value === "Pokhara";
});

The result should look like

0: Kathmandu
1: Pokhara

What happened here? Since the third iteration returned true, we successfully stopped the loop!

Array.prototype.every

Array.prototype.every is almost identical to some except it’s expecting false to break the loop.

Example:

var arr = ["Kathmandu", "Pokhara", "Lumbini", "Gorkha"];
 
arr.every(function(value, index, _arr) {
console.log(index + ": " + value);
return value.indexOf("Lumbini") &lt; 0;
});