The For In Loop

The JavaScript for in statement loops through the properties of an object.

Syntax

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

Example‍


Example Explained

• The for in loop iterates over the car object.
• Each iteration returns a property key (prop).
• The key is used to access the corresponding value (car[prop]).

For In Over Arrays

The JavaScript for in statement can also loop over the properties of an array.

Syntax

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for (variable in array) {
 // code block to be executed
}


Example

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const fruits = ["Apple", "Banana", "Cherry"];

let list = "";
for (let index in fruits) {
 list += fruits[index] + " ";
}


Note

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.

Array.forEach()

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

Example

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const numbers = [1, 2, 3, 4, 5];

let result = "";
numbers.forEach(addNumber);

function addNumber(value) {
 result += value + " ";
}


Note

The function passed to forEach() takes three arguments:

• The item value
• The item index
• The array itself

The example above uses only the value parameter.

Revised Example

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const numbers = [10, 20, 30, 40, 50];

let sum = 0;
numbers.forEach(calculateSum);

function calculateSum(value) {
 sum += value;
}


This revised example calculates the sum of the numbers in the array.

Summary

• Use for in to loop through the properties of an object.
• Use for in with caution on arrays, as the order may not be guaranteed.
• Prefer for, for of, or Array.forEach() when the order of elements in an array is important.

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