JS Loop For In

The For In Loop

The JavaScript for/in statement iterates over the properties of an object.

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

const person = {fname:“John”, lname:“Doe”, age:25};

let text = “”;
for (let x in person) {
text += person[x];
}

The for/in loop iterates over a person object.
In each iteration, it returns a key (x), which is then used to access the corresponding value.
The value can be accessed using person[x].

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

for (variable in array) {
code
}

const numbers = [45, 4, 9, 16, 25];

let txt = “”;
for (let x in numbers) {
txt += numbers[x];
}

Avoid using for/in with an array if the index order matters.

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

It’s better to use a for loop, a for/of loop, or Array.forEach() when maintaining order is important.

The forEach() method invokes a callback function once for each element in the array.

const numbers = [45, 4, 9, 16, 25];

let txt = “”;
numbers.forEach(myFunction);

function myFunction(value, index, array) {
txt += value;
}

Note that the function accepts three arguments:

The example above uses only the value parameter and can be rewritten to include the others:

const numbers = [45, 4, 9, 16, 25];

let txt = “”;
numbers.forEach(myFunction);

function myFunction(value) {
txt += value;
}