What are the pros/cons of iterating an array with Array.prototype.forEach() versus for() in Javascript

Question

What are the pros/cons of iterating an array with Array.prototype.forEach() versus for() in Javascript?

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

Answer 1

forEach looks better sometimes and can avoid intermediate variables and closure problems.

for is faster, works on non-arrays, maintains this by default, and has always been part of JavaScript. You can also break out of it with break; — even nested.

In ES6, forEach’s this-maintaining and looks are improved by the introduction of =>; for’s verbosity and sometimes unintuitive scope is improved by let and for of loops.

As 6502 answered, they work differently on sparse arrays.
Do not use sparse arrays.

Answer 2

They do two very different things:

1. for (var i=0; i<L.length; i++) { ... } iterates over ALL indexes, including array elements that "are undefined".
2. forEach iterates only on defined elements.

Note also that being "undefined" doesn't simply mean that the value of an element is undefined, but that the element as never been set and the two are very different things:

a = [];
a[3] = 9;
a.indexOf(undefined) // --> -1
a[1] = undefined;
a.indexOf(undefined) // --> 1

so you can have a defined element with value undefined...

a = [];
a[9] = 9;
a[3] = undefined;

a.forEach(function(){ console.log(arguments); });

will show two lines, one for element with index 9 and one for element with index 3