JavaScript for...in Loop

Iterate (loop) over the properties of an object:

const person = {fname:"John", lname:"Doe", age:25};
let text = "";
for (let x in person) {
text += person[x] + " ";
}

Iterate (loop) over the values of an array:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let text = "";
for (let x in cars) {
text += cars[x] + " ";
}

More examples below.

Description

The for...in statements combo iterates (loops) over the properties of an object.

The code block inside the loop is executed once for each property.

Do not use for...in to iterate an array if the index order is important. Use a for loop instead.

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

Parameter	Description
x	Required. A variable to iterate over the properties.
object	Required. The object to be iterated

Statement	Description
break	Breaks out of a loop
continue	Skips a value in a loop
while	Loops a code block while a condition is true
do...while	Loops a code block once, and then while a condition is true
for	Loops a code block while a condition is true
for...of	Loops the values of any iterable
for...in	Loops the properties of an object

Iterate over the properties of window.location:

let text = "";
for (let x in location) {
text += x + "
";
}
document.getElementById("demo").innerHTML = text;

for...in is an ECMAScript1 (ES1) feature.

ES1 (JavaScript 1997) is fully supported in all browsers:


Chrome	Edge	Firefox	Safari	Opera	IE
Yes	Yes	Yes	Yes	Yes	Yes