Possible strange output of javascript forEach loop

Question

How does this output make any sense? Maybe I am thinking about it wrong but it seems fairly descriptive in what it's supposed to be dong

var dataset = [1, 2, 3];

dataset.forEach(function(element, index, array) {
    //          (index, how many to remove)
    array.splice(0, 0);
});

returns [1,2,3] as expected

dataset.forEach(function(element, index, array) {
    array.splice(0, 3);
});

returns [] as expected

dataset.forEach(function(element, index, array) {
   array.splice(0, 1);
});

returns [3]

dataset.forEach(function(element, index, array) {
    array.splice(0, 2);
});

returns [3]

This has got me questioning my understanding of everything lol. I had a more complex requirement where an array of objects may have property "element.archived"

var dataset = [Object, Object, Object]

dataset.forEach(function(element, index, array) {
   if (element.archived) array.splice(index, 1);
});

I am trying to iterate over the array, and remove all objects that have this property value of true.

methinks you just use wrong instrument for filtering. seems you need simple array.filter — Grundy
– Grundy, Commented Dec 2, 2015 at 17:00
Why are you doing array.splice inside the forEach? Do you really need to run splice on the array 3 times? — gen_Eric
– gen_Eric, Commented Dec 2, 2015 at 17:02
@RocketHazmat, exactly what my question was going to be. OP you seem to have a misunderstanding of how splice works. — Andy
– Andy, Commented Dec 2, 2015 at 17:04
splice modifies indices, forEach starts from 0 and counts up. When it reaches a non-defined index it stops. Because the length changed, the non-defined index is earlier. Therefore forEach is invoked less than initial length times — Paul S.
– Paul S., Commented Dec 2, 2015 at 17:19
@JasonMacAnLighiche: Do not edit your solution into the question. The question should remain a question. What you should do is accept the correct answer. — gen_Eric
– gen_Eric, Commented Dec 2, 2015 at 18:50

Andy · Accepted Answer · 2015-12-02 17:27:33Z

2

So you want to remove objects from an array that has archived: true. Here's a couple of ways to do this:

You can use filter which returns a new filtered array:

var arr = dataset.filter(function (el) {
  return el.archived !== true;
});

Or you can use splice to remove elements as you iterate over them:

for (var i = dataset.length - 1; i >= 0; i--) {
  if (dataset[i].archived === true) dataset.splice(i, 1);
}

DEMO

edited Dec 2, 2015 at 17:27

answered Dec 2, 2015 at 17:11

Andy

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4 Comments

Grundy Over a year ago

seems sample with splice wrong - when we remove item we should not increment i variable

Andy Over a year ago

Doh. Thanks for the headsup.

Paul S. Over a year ago

@Andy instead of decrementing i for each match or "not incrementing" it, it is actually easier to loop downwards instead as these indices won't be changed var i = dataset.length; while (i-- > 0) {...}

Andy Over a year ago

@PaulS., thanks. Sometimes you repeat code enough times it becomes habit and never think there might be a better way of writing it until someone points it out

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