TypeScript for

Summary: in this tutorial, you will learn about the TypeScript for loop statement that executes a piece of code repeatedly.

Introduction to the TypeScript for statement

The following shows the syntax of the TypeScript for loop statement:

for(initialization; condition; expression) {
    // statement
}Code language: TypeScript (typescript)

The for loop statement creates a loop. It consists of three optional expressions separated by semicolons (;) and enclosed in parentheses:

• initialization: is an expression evaluated once before the loop begins. Typically, you use the initialization to initialize a loop counter.
• condition – is an expression that is evaluated at the end of each loop iteration. If the condition is true, the statements in the loop body execute.
• expression – is an expression that is evaluated before the condition is evaluated at the end of each loop iteration. Generally, you use the expression to update the loop counter.

All three expressions in the for loop statement are optional. It means that you can use the for loop statement like this:

for(;;) {
    // do something
}Code language: TypeScript (typescript)

In practice, you should use a for loop if you know how many times the loop should run. If you want to stop the loop based on a condition other than the number of times the loop executes, you should use a while loop.

TypeScript allows you to omit the loop body completely as follows:

for(initialization; condition; expression);Code language: TypeScript (typescript)

However, it is rarely used in practice because it makes the code more difficult to read and maintain.

TypeScript for examples

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

1) Simple TypeScript for example

The following example uses the for loop statement to output 10 numbers from 0 to 9 to the console:

for (let i = 0; i < 10; i++) {
    console.log(i);
}Code language: TypeScript (typescript)

Output:

0
1
2
3
4
5
6
7
8
9Code language: TypeScript (typescript)

How it works:

• First, declare a variable i and initialize it to 0.
• Then check if i is less than 10. If it is, output i to the console and increment the variable i by one.
• Finally, repeat the second step until i equals 10.

2) TypeScript for example: optional block

The following example shows the same output as the above example. However, the for doesn’t have the initialization block:

let i = 0;
for (; i < 10; i++) {
    console.log(i);
}Code language: TypeScript (typescript)

Like the initialization block, you can omit the condition block.

However, you must escape the loop when a condition is met by using the if and break statements. Otherwise, you will create an infinite loop that causes the program to execute repeatedly until it is crashed.

for (let i = 0; ; i++) {
    console.log(i);
    if (i > 9) break;
}Code language: TypeScript (typescript)

The following example illustrates a for loop that omits all three blocks:

let i = 0;
for (; ;) {
    console.log(i);
    i++;
    if (i > 9) break;
}Code language: TypeScript (typescript)

Output:

0
1
2
3
4
5
6
7
8
9Code language: TypeScript (typescript)

How it works:

• First, declare a loop counter i and initialize it to 0 before entering the for.
• Then, in each loop iteration, output i to the console, increment it by one, and break out of the loop if i is greater than 9.

Summary

• Use the TypeScript for statement when you want to repeatedly execute a piece of code a number of times.