TypeScript for Loop

Note: If you're new to TypeScript, check our Getting Started with TypeScript tutorial first.

The TypeScript for loop is used to iterate over a block of code a certain number of times or to iterate over the elements of an array.

for (let i: number = 0; i < 3; i++) {
    console.log("Hello, world!");
}

// Output:
// Hello, world!
// Hello, world!
// Hello, world!

TypeScript for loop Syntax

The syntax of the for loop is:

for (initialExpression; condition; updateExpression) {
    // Body of for loop
}

Here,

• initialExpression - Initializes a counter variable.
• condition - The condition to be evaluated. If true, the body of the for loop is executed.
• updateExpression - Updates the value of initialExpression.

Once an iteration of the loop is completed, the condition is evaluated again. The process continues until the condition is false.

To learn more about the condition, visit TypeScript Comparison and Logical Operators.

Flowchart of TypeScript for Loop

Example 1: Print Numbers From 1 to 5

for (let i: number = 1; i < 6; i++) {
    console.log(i);
}

Output

1
2
3
4
5

In this example, we have printed numbers from 1 to 5 using a for loop. In each iteration of the loop, the value of the variable i is printed to the console.

Here's how this program works in each iteration of the loop:

Example 2: Display Sum of n Natural Numbers

// Program to display the sum of natural numbers

let sum: number = 0;
const n: number = 100

// Loop from i = 1 to i = n
// in each iteration, i is increased by 1
for (let i: number = 1; i <= n; i++) {
    sum += i;  // sum = sum + i
}

console.log(`Sum: ${sum}`);

// Output: Sum: 5050

Initially, the value of sum is 0, while n has a constant value of 100.

Then, we iterate a for loop from i = 1 to n. In each iteration,

• i is added to sum.
• Then, the value of i is increased by 1.

When i becomes 101, the test condition becomes false and sum will be equal to 0 + 1 + 2 + ... + 100.

Iterate Through an Array

A for loop can also be used to iterate over elements of an array.

Note: An array is a collection of data of the same type. To learn more, visit TypeScript Array.

Example

const fruits: string[] = ["apple", "banana", "cherry"];

for (let i: number = 0; i < fruits.length; i++) {
    console.log(fruits[i]);
}

Output

apple
banana
cherry

This loop iterates through the fruits array and prints each element to the console.

// Outer loop 
for (let i: number = 0; i < 3; i++) {

    // Inner loop
    for (let j: number = 0; j < 2; j++) {
        console.log(`i = ${i}, j = ${j}`);
    }
}

i = 0, j = 0
i = 0, j = 1
i = 1, j = 0
i = 1, j = 1
i = 2, j = 0
i = 2, j = 1

for (let i: number = 0; true; i++) {
    console.log("This loop will run forever!");
}

This loop will run forever!
This loop will run forever!
This loop will run forever!
…

// Initialization outside the loop
let i: number = 0;

// Omit initialization and update statements
for (; i < 3; ) {

    console.log(`i is ${i}`);

    // Increment inside the loop body
    i++; 
}

i is 0
i is 1
i is 2

Also Read:

• TypeScript while and do...while Loop
• TypeScript break Statement
• TypeScript continue Statement