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JavaScript For In


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

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

Syntax

for (variable in array) {
  code
}



Example

const numbers = [45, 4, 9, 16, 25];
let txt = "";
for (let x in numbers) {
  txt += numbers[x];
}

Try it Yourself »


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

const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value, index, array) {
 
txt += value;
}

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Note that the function takes 3 arguments:

The item value
The item index
The array itself

The example above uses only the value parameter. It can be rewritten to:

Example

const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value) {
 
txt += value;
}

Try it Yourself »













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Reference: https://www.w3schools.com/js/js_loop_forin.asp