JavaScript - For Loop

JavaScript for Loop

The for loop executes a set of statements in each iteration. Whenever, number of iteration is known, for loop is preferred over while loop.

Syntax

for(initialization(s); condition(s); counter_update(s);){
  statements;
}

Initialization(s): Variable(s) is initialized in this section and executed for one time only.
Condition(s): Condition(s) is defined in this section and executed at the start of loop everytime.
Counter Update(s): Loop counter(s) is updated in this section and executed at the end of loop everytime.

Flow Diagram:

Example:

In the example below, the program continue to print variable called i from value 1 to 5. After that, it exits from the for loop.

var txt = "";

for (i = 1; i <= 5; i++){
  txt = txt + "i = " + i + "<br>";

The output (value of txt) after running above script will be:

i = 1
i = 2
i = 3
i = 4
i = 5

Example

Multiple initializations, condition checks and loop counter updates can be performed in a single for loop. In the example below, two variable called i and j are initialized, multiple conditions are checked and multiple variables are updated in a single for loop.

var txt = "";

for (i = 1, j = 100;   i <= 5 || j <= 800;   i++, j = j + 100)
  txt = txt + "i = " + i + ", j = " + j +"<br>";

The output (value of txt) after running above script will be:

i = 1, j = 100
i = 2, j = 200
i = 3, j = 300
i = 4, j = 400
i = 5, j = 500
i = 6, j = 600
i = 7, j = 700
i = 8, j = 800