JavaScript For Loop

Looping in programming languages is a feature that facilitates the execution of a set of instructions repeatedly until some condition evaluates and becomes false. We come across for loop which provides a brief and systematic way of writing the loop structure. Syntax:

for (statement 1 ; statement 2 ; statement 3){
        code here...
        }

• Statement 1: It is the initialization of the counter. It is executed once before the execution of the code block.
• Statement 2: It is the testing statement that defines the condition for executing the code block It must return a boolean value. It is also an entry-controlled loop as the condition is checked before the execution of the loop statements.
• Statement 3: It is the increment or decrement of the counter & executed (every time) after the code block has been executed.

Example:

javascript

<script type="text/javascript" charset="utf-8">
    // JavaScript program to illustrate for loop
           var x;

           // for loop begins when x=2
           // and runs till x <=4
           for (x = 2; x <= 4; x++)
           {
           console.log("Value of x:" + x);
           }
</script>

Output:

Value of x:2
Value of x:3
Value of x:4

Flow chart

Statement 1: We can initialize the counter variable externally rather than in statement 1. This shows us clearly that statement 1 is optional. We can leave the portion empty with a semicolon. For example: 

javascript

<script>
    var x = 2;

    for ( ; x <= 4; x++)
    {
        console.log("Value of x:" + x);
    }
</script>

Output:

Value of x:2
Value of x:3
Value of x:4

Statement 2: This statement checks the boolean value of the testing condition. If the testing condition is true, the for loop will execute further. It is executed every time the for loop runs before the loop enters its body. If this statement returns true, the loop will start over again, otherwise, it will end and move over to the code following the loop body. This is also an optional statement and Javascript treats it as true if left blank. If this statement is omitted, the loop runs indefinitely if the loop control isn’t broken manually. It is explained below: 

Javascript

<script>
    var x = 2;
        for ( ; ; x++)
        {
            console.log("Value of x:" + x);
            break;
        }
</script>

Output:

Value of x: 2

Statement 3: It is a controlled statement that controls the increment/decrement of the counter variable. It is also optional by nature and can be done inside the loop body. For example:- 

javascript

<p id="gfg"></p>
<script>
    const subjects = ["Maths", "Science", "Polity", "History"];
    var i = 0;
    var len = subjects.length;
    var gfg = "";
    for (; i < len; ) {
    gfg += subjects[i+'']+"<br>" ;
    //can be increased inside loop
    i++;
    }
    document.getElementById('gfg').innerHTML = gfg
</script>

Output:

Maths
Science
Polity
History

for/in loop: There is another advanced loop called for/in loop which runs through all the properties of an object. The loop will be executed once for each property of the object.

Syntax :

 for (var in object) { statements to be executed } 

Example:

javascript

<button onclick="GFG()">Try it</button>
<p id="gfg"></p>

<script>
    function GFG() {
    var Platform= {fname:"geeks", Mname:"for", lname:"geeks", };

    var text = "";
    var x;
    for (x in Platform) {
        text += Platform[x] + " ";
    }
    document.getElementById("gfg").innerHTML = text;
    }
</script>

Output:

The for/in loop can also be used over the properties of arrays. However, it is not advised to use for/in loop over arrays.  for/of and Array.forEach() loops are suggested to be used while working with arrays.

Syntax:

for(var in array){statements to be executed}

Example: 

Javascript

<script type="text/javascript" charset="utf-8">
    const arr = [23, 54, 46, 3];

    let gfg = "";
    for (let i in arr) {
    gfg += arr[i] +'\n';
    }
      console.log(gfg)
</script>

Output:

23
54
46
3

We have a Cheat Sheet on JavaScript where we covered all the important topics of JavaScript to check those please go through JavaScript Cheat Sheet – A Basic Guide to JavaScript.