TL;DR

• Your best bets are usually
  • a for-of loop (ES2015+ only; spec | MDN) – simple and async-friendly

    for (const element of theArray) {
        // ...use element...
    }
    

  • forEach (ES5+ only; spec | MDN) (or its relatives some and such) – not async-friendly (but see details)

    theArray.forEach(element => {
        // ...use element...
    });
    

  • a simple old-fashioned for loop – async-friendly

    for (let index = 0; index < theArray.length; ++index) {
        const element = theArray[index];
        // ...use element...
    }
    

  • (rarely) for-in with safeguards – async-friendly

    for (const propertyName in theArray) {
        if (/*...is an array element property (see below)...*/) {
            const element = theArray[propertyName];
            // ...use element...
        }
    }
    

• Some quick “don’t”s:
  • Don’t use for-in unless you use it with safeguards or are at least aware of why it might bite you.
  • Don’t use map if you’re not using its return value.
    (There’s sadly someone out there teaching map [spec / MDN] as though it were forEach — but as I write on my blog, that’s not what it’s for. If you aren’t using the array it creates, don’t use map.)
  • Don’t use forEach if the callback does asynchronous work and you want the forEach to wait until that work is done (because it won’t).

But there’s lots more to explore, read on…

JavaScript has powerful semantics for looping through arrays and array-like objects. I’ve split the answer into two parts: Options for genuine arrays, and options for things that are just array-like, such as the arguments object, other iterable objects (ES2015+), DOM collections, and so on.

Okay, let’s look at our options:

For Actual Arrays

You have five options (two supported basically forever, another added by ECMAScript 5 [“ES5”], and two more added in ECMAScript 2015 (“ES2015”, aka “ES6”):

1. Use for-of (use an iterator implicitly) (ES2015+)
2. Use forEach and related (ES5+)
3. Use a simple for loop
4. Use for-in correctly
5. Use an iterator explicitly (ES2015+)

(You can see those old specs here: ES5, ES2015, but both have been superceded; the current editor’s draft is always here.)

Details:

1. Use for-of (use an iterator implicitly) (ES2015+)

ES2015 added iterators and iterables to JavaScript. Arrays are iterable (so are strings, Maps, and Sets, as well as DOM collections and lists, as you’ll see later). Iterable objects provide iterators for their values. The new for-of statement loops through the values returned by an iterator:

const a = ["a", "b", "c"];
for (const element of a) { // You can use let instead of const if you like
    console.log(element);
}
// a
// b
// c