The For In Loop

The JavaScript for in statement loops through the properties of an Object:


for (key in object) {
  // code block to be executed

Example Explained:

The for in loop iterates over a person object

Each iteration returns a key (x)

The key is used to access the value of the key

The value of the key is person[x]


For In Over Arrays

The JavaScript for in statement can also loop over the properties of an Array:


for (variable in array) {
  // code

Do not use for in over an Array if the index order is important.

The index order is implementation-dependent, and array values may not be accessed in the order you expect.

It is better to use a for loop, a for of loop, or Array.forEach() when the order is important.



The forEach() method calls a function (a callback function) once for each array element.

Note that the function takes 3 arguments:

The item value

The item index

The array itself