JavaScript for Loop

Summary: in this tutorial, you will learn how to use the JavaScript for loop statement to create a loop with various options.

Introduction to the JavaScript for loop statement

The for loop statement creates a loop with three optional expressions. The following illustrates the syntax of the for loop statement:

for (initializer; condition; iterator) {
    // statements
}Code language: JavaScript (javascript)

1) initializer

The for statement executes the initializer only once the loop starts. Typically, you declare and initialize a local loop variable in the initializer.

2) condition

The condition is a boolean expression that determines whether the for should execute the next iteration.

The for statement evaluates the condition before each iteration. If the condition is true (or is not present), it executes the next iteration. Otherwise, it’ll end the loop.

3) iterator

The for statement executes the iterator after each iteration.

The following flowchart illustrates the for loop:

In the for loop, the three expressions are optional. The following shows the for loop without any expressions:

for ( ; ; ) {
   // statements
}Code language: JavaScript (javascript)

JavaScript for loop examples

Let’s take some examples of using the for loop statement.

1) A simple JavaScript for loop example

The following example uses the for loop statement to show numbers from 1 to 4 to the console:

for (let i = 1; i < 5; i++) {
  console.log(i);
}
Code language: JavaScript (javascript)

Output:

1
2
3
4

How it works.

• First, declare a variable  counter and initialize it to 1.
• Second, display the value of counter in the console if counter is less than 5.
• Third, increase the value of counter by one in each iteration of the loop.

2) Using the JavaScript for loop without the initializer example

The following example uses a for loop that has no initializer expression:

let j = 1;
for (; j < 10; j += 2) {
  console.log(j);
}
Code language: JavaScript (javascript)

Output:

1
3
5
7
9

3) Using the JavaScript for loop without the condition example

Similar to the initializer expression, the condition expression is optional. If you omit the condition expression, you need to use a break statement to terminate the loop.

for (let j = 1; ; j += 2) {
  console.log(j);
  if (j > 10) {
    break;
  }
}
Code language: JavaScript (javascript)

Output:

1
3
5
7
9
11

3) Using the JavaScript for loop statement without any expression example

All three expressions of the for loop statements are optional. Therefore, you can omit all of them. For example:

let j = 1;
for (;;) {
  if (j > 10) {
    break;
  }
  console.log(j);
  j += 2;
}
Code language: JavaScript (javascript)

Output:

1
3
5
7
9

4) Using the JavaScript for loop without the loop body example

JavaScript allows the for statement to have an empty statement. In this case, you place a semicolon (;) immediately after the for statement.

For example, the following uses a for loop to calculate the sum of 10 numbers from 1 to 10:

let sum = 0;
for (let i = 0; i <= 9; i++, sum += i);
console.log(sum);Code language: JavaScript (javascript)

Output:

55

Summary

• Use the JavaScript for statement to create a loop that executes a block of code using various options.