Javascript - how to start forEach loop at some index

Question

I have an array with alot of items, and I am creating a list of them. I was thinking of paginating the list. I wonder how can I start a forEach or for

Answer 1

forEach doesn't offer that feature, no. So your choices are:

1. A simple for loop

2. Ignoring the indexes you don't want to handle (as in Kind user's answer)

3. Using slice (as in Nina's answer)

4. Writing your own function

Here's #4 as an Array.prototype extension (non-enumerable, of course; adding enumerable properties to Array.prototype breaks a lot of code); after it is a standalone version for when adding to Array.prototype isn't appropriate:

// Giving ourselves the function
Object.defineProperty(Array.prototype, "myEach", {
  value: function(from, to, callback, thisArg) {
    if (typeof from === "function") {
      thisArg = callback;
      callback = to;
      to = from;
      from = 0;
    }
    if (typeof to === "function") {
      thisArg = callback;
      callback = to;
      to = this.length;
    }
    for (var n = from; n < to; ++n) {
      callback.call(thisArg, this[n], n, this);
    }
  }
});

// Using it:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
arr.myEach(3, function(e) { console.log(e); });
console.log("*** From 3 (inclusive) to 5 (exclusive):");
arr.myEach(3, 5, function(e) { console.log(e); });
console.log("*** All:");
arr.myEach(function(e) { console.log(e); });
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
arr.myEach(0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}

Again note that that's a non-enumerable property, which is vital if you ever add anything to Array.prototype (otherwise, you break a lot of code).

You wouldn't do that in a library to be consumed by others, you'd just have a standalone function:

// Giving ourselves the function
function myEach(array, from, to, callback, thisArg) {
  if (typeof from === "function") {
    thisArg = callback;
    callback = to;
    to = from;
    from = 0;
  }
  if (typeof to === "function") {
    thisArg = callback;
    callback = to;
    to = array.length;
  }
  for (var n = from; n < to; ++n) {
    callback.call(thisArg, array[n], n, array);
  }
}

// Using it:
var arr = ["zero", "one", "two", "three", "four", "five", "six", "seven"];
console.log("*** From 3:");
myEach(arr, 3, function(e) {
  console.log(e);
});
console.log("*** From 3 (inclusive) to 5 (exclusive):");
myEach(arr, 3, 5, function(e) {
  console.log(e);
});
console.log("*** All:");
myEach(arr, function(e) {
  console.log(e);
});
console.log("*** Check thisArg handling on 0-2:");
var o = {answer: 42};
myEach(arr, 0, 2, function(e) {
  console.log(e + " (this.answer = " + this.answer + ")");
}, o);

.as-console-wrapper {
  max-height: 100% !important;
}