[austincheney]

In cases where order doesn't matter you can avoid you can avoid the array length question all together by decrementing:

    let index = arr.length;
    do {
        index -= 1;
        console.log(arr[index]);
    } while (index > 0);

As a side note whether it takes longer to access a variable or object property is largely superficial depending upon the size of object because it implies creating a new variable on which to store that object property. There is time involved to invoke a new variable just as there is time involved to access an object's property.

As an added bit of trivia in the 1970s a software developer named Paul Heckel, known for Heckel Diff algorithm, discovered that access to object properties is faster than accessing array indexes half the time. That was in C language, but it holds true in JavaScript.

[nostrademons]

In 2009 we standardized on this form for all Google Search JS for-each loops, because we were literally counting bytes on the SRP:

  for(var i=0,e;e=a[i++];){ ... }

Could result in problems if the array contained falsey values, but we just didn't do that.

Nowadays, like I mentioned above, I'd just do

  arr.foreach((elem, i) => { ... });

Which last time I checked was significantly slower than the for-loop, but I've learned my lesson about trying to optimize for browser quirks that may disappear in a year or two. :-)

[layoutIfNeeded]

>[...] discovered that access to object properties is faster than accessing array indexes half the time. That was in C language, but it holds true in JavaScript.

What are these object properties in C?

[austincheney]

Hash map then.

[layoutIfNeeded]

Hm. So you're saying that indexing into a hash map can be faster than indexing into an array? How would this be possible? I mean, under the hood a hash map is going to an array too, which is being indexed based on the hash value...

[austincheney]

Hash map properties are randomly accessed from memory opposed to array indexes.