There are several approaches to iteration:

    • for, while, and do while loops
    • for ... in and for ... of loops for object iteration
    • forEach for array iteration (and map, reduce, etc. for array transformation)

    Generally, forEach and array transformations are slightly more idiomatic than loops, since they express the developer's intent more clearly. However, loops perform faster.


    JavaScript supports for, while, and do while loops.

    We can use break to exit the loop immediately or continue to advance to the next iteration. Nested loops can be labeled.

    These have essentially the same syntax and work the same way as in other C-like languages.

    For ... of

    We can use a for ... of loop to iterate through values within an object.

    We do this most commonly with arrays, but many standard library objects can be iterated this way.

    We can also define custom iteration behavior.

    For ... in

    The for ... in loop iterates through the keys of an object.

    However, it also includes inherited properties, which is often not what we want.

    Due to historical complexities, for ... in loops aren't very common.


    If we do want to use a for ... in loop, we can use the hasOwnProperty method to filter inherited properties.

    Modifying the prototype of a class will modify key lookup behavior for every object, and is something you should almost never do. However, it's possible that a library may do it (possibly by mistake), so it's still something to watch out for.


    We use forEach to call a function for each element of an array. The function is also passed the element, the element's index, and the array itself.

    We'll generally use this instead of for ... of, unless we need to exit the loop early (i.e. break doesn't exist in forEach loops).