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
}
// code block to be executed
}
Example
var person = {fname:"John", lname:"Doe", age:25};
var text = "";
var x;
for (x in person) {
text += person[x];
}
var text = "";
var x;
for (x in person) {
text += person[x];
}
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
}
code
}
Example
var numbers = [45, 4, 9, 16, 25];
var txt = "";
var x;
for (x in numbers) {
txt += numbers[x] + "<br>";
}
document.getElementById("demo").innerHTML = txt;
var txt = "";
var x;
for (x in numbers) {
txt += numbers[x] + "<br>";
}
document.getElementById("demo").innerHTML = txt;
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
var txt = "";
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
function myFunction(value, index, array) {
txt = txt + value + "<br>";
}
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
function myFunction(value, index, array) {
txt = txt + value + "<br>";
}
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
The example above uses only the value parameter. The example can be rewritten to:
Example
var txt = "";
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
function myFunction(value) {
txt = txt + value + "<br>";
}
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);
function myFunction(value) {
txt = txt + value + "<br>";
}
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