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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:

JavaScript For Loop

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 of the above code 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

JavaScript for-each Loop

JavaScript for-each loop is used to traverse an array or a collection. In each iteration of the loop, current element is assigned to a user defined variable and array/collection pointer is moved to next element which is processed in the next iteration. Please note that it works on element basis (not index basis).

Syntax

for(variable: array/collection){
  statements;
} 

In the example below, the foreach loop is used on array called Arr which is further used to print each element of the array in each iteration.

Arr = {10, 20, 30, 40 ,50};
var txt;

    //System.out.println("The Arr contains:");
for(i: Arr) 
  txt = i + "<br>";

The output of the above code will be:

The Arr contains:
10
20
30
40
50

5