In this lesson, you'll learn about and loops in JavaScript, which are essential for repeating actions multiple times efficiently.

For and While Loops

Sometimes we want to repeat an action a number of times. For example, let’s imagine we want to display numbers from zero to nine on the console. You might think of doing something like this:

console.log(0);
console.log(1);
console.log(2);
console.log(3);
console.log(4);
console.log(5);
console.log(6);
console.log(7);
console.log(8);
console.log(9);

But that’s not a good idea at all. Instead, we use or loops.

Loop

The loop is more complex, but it’s also the most commonly used loop. It’s called a loop because it runs "for" a specific number of times .

loops are declared with three optional expressions separated by semicolons: initialization, condition, and final-expression, followed by a statement (usually a block statement).

for ([initialization]; [condition]; [final-expression]) {
    // statement
}

• Initialization: Executed once before the loop starts. It's typically used to define and set up your loop variable.
• Condition: Evaluated at the beginning of every loop iteration and will continue as long as it evaluates to . If the condition is at the start of the iteration, the loop will stop executing.
• Final-expression: Executed at the end of each loop iteration, prior to the next condition check, and is usually used to increment or decrement your loop counter.

Let's use a loop to print numbers from zero to nine:

for (let i = 0; i < 10; i++) {
    console.log(i);
}

• Initialization: We initialize our variable because we start counting from 0. "i" stands for index and is a standard for loop variables.
• Condition: We set our condition to . Before each loop execution, it checks if is less than ten. If is equal to or greater than 10, the loop terminates.
• Final-expression: We use , which is shorthand for . Each iteration increases by one.

Optional Expressions in Loop

Expressions in a loop are optional, meaning we can skip parts. For example, we can initialize the loop variable before the loop like this:

let i = 0; // we have i already declared and assigned
for (; i < 10; i++) { // no need for "initialization"
    console.log(i); // 0, 1, 2, ..., 9
}

We can even remove everything, creating an infinite loop :

for (;;) {
    // repeats without limits
}

Loop

The loop is simpler than the loop and is used when the number of iterations is not known beforehand. It continues to execute a block of code as long as a specified condition evaluates to .

while (condition) {
    // statement
}

• Condition: Evaluated before each loop iteration. If it evaluates to , the loop continues; if , the loop stops.

Let's use a loop to print numbers from zero to nine:

let i = 0;
while (i < 10) {
    console.log(i);
    i++;
}

• Initialization: We initialize before the loop starts.
• Condition: The loop continues as long as .
• Increment: We manually increment within the loop using .

Infinite Loop

A loop can also become an infinite loop if the condition never evaluates to . Be cautious to ensure that the loop has a way to terminate:

while (true) {
    // This will run forever unless there's a break statement
}

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