Breaking or continuing loop in functional programming

A common requirement of iteration is cancelation. Using for loops we can break to end iteration early.

const a = [0, 1, 2, 3, 4];
for (var i = 0; i < a.length; i++) {
  if (a[i] === 2) {
    break; // stop the loop
  }
  console.log(a[i]);
}
//> 0, 1

Another common requirement is to close over our variables.

A quick approach is to use .forEach but then we lack the ability to break. In this situation the closest we get is continue functionality through return.

[0, 1, 2, 3, 4].forEach(function(val, i) {
  if (val === 2) {
    // how do we stop?
    return true;
  }
  console.log(val); // your code
});
//> 0, 1, 3, 4

The .some is a method on Array prototype. It tests whether some element in the array passes the test implemented by the provided function. If any value is returning true, then it stops executing. Here is a MDN link for more details.

An example quoted from that link

const isBiggerThan10 = numb => numb > 10;

[2, 5, 8, 1, 4].some(isBiggerThan10);  // false
[12, 5, 8, 1, 4].some(isBiggerThan10); // true

Using .some we get iteration functionally similar to .forEach but with the ability to break through return instead.

[0, 1, 2, 3, 4].some(function(val, i) {
  if (val === 2) {
    return true;
  }
  console.log(val); // your code
});
//> 0, 1

You keep returning false to make it continue to next item. When you return true, the loop will break and a.some(..) will return true.

// Array contains 2
const isTwoPresent = [0, 1, 2, 3, 4].some(function(val, i) {
  if (val === 2) {
    return true; // break
  }
});
console.log(isTwoPresent);
//> true