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JavaScript JavaScript Loops, Arrays and Objects Tracking Data Using Objects Using `for in` to Loop Through an Object's Properties

Robert Mews
Robert Mews
11,540 Points

For and For in loops - Arrays and Objects?

Hey all,

This is my future self going back trying to get clarity around for and for in loops for mixing arrays and objects.

It seems you'd use a for loop if you have objects inside an array, but use a for in loop if you have an array inside an object. Does anyone have a clear way to explain why?

1 Answer

This is a very simplistic explanation, but that's because I understand the difference between for and for/in at a very superficial level :-)

A 'for' loop gives you the a pretty open framework to iterate over the array in steps from beginning to end, a portion of the array, or skipping (i = 0; i < array.length; i += 2). You don't need to iterate over every index (analagous to 'property' of an object), just the ones you select when setting up the range in the for statement. But in any case, an array's properties (or "indices") are numbered from 0 to whatever. And each of the elements in the array can be numbers, strings, arrays, objects or functions.

A 'for/in' loop is meant to iterate over EVERY property in the object, which usually has very different names for each property, such as "firstName", "lastName", "email", "city", etc. And the value associated with each property in the object can be a number, string, array, object, or function.