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

When do we need to use for-in loop?

Hi guys, could you explain me better which is the difference between a normal loop and a for-in loop? It's difficult to understand which are the situations when to use one instead of the other one. Thanks for the clarification! Luca

Idan Melamed
Idan Melamed
16,285 Points

Hi Luca, I'm just taking this course as you, so I'm not sure my answer will be 100% correct.

A for loop looks like this:

for ( var i = 0; i  < 10; i += 1 ) {

In this example the loop will run exactly 10 times. So regular loops are great for when you know exactly how many times you need to run the through the loop. It is really good for going through arrays, because the first item in an array is array[0], the second item is array[1] and so on... And you can always use the 'length' property of an array to serve as your condition. Like this:

var squares = [1, 4, 9, 16, 25, 36];
for ( var i = 0; i < squares.length; i += 1 ) {

The for...in loop goes through the properties of objects. For example, if you want to log all the values of:

var shoppingCart = { 
  size: 'small',
  color: 'green', 
  quantity: 17

You can do it with:

for ( var prop in shoppingCart ) {
  console.log( shoppingCart[prop] );

So regular for loops are great to use when you know exactly how many times you want to run it, and when you want to go through arrays.

For...in loops are great when you want to go through objects. You didn't even have to know how long the object is!

2 Answers

Steven Parker
Steven Parker
228,991 Points

Idan's right about for..in being handy for objects and properties. But it's also good for arrays.

For example, his array loop example that uses .length can be re-written this way:

var squares = [1, 4, 9, 16, 25, 36];
for ( var i in squares ) {

So I think it's always useful to use for... in loops, ins't it?