Iteration

There are a number of different ways to loop or iterate in JavaScript: for, while, do ... while

For Loops

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A for loop has the following general structure:

for (initialisation; condition; final - expression) {
    // ... code to execute on each iteration
}

• The initialisation is where we set up the variable used in iteration (a counter for instance)
• The condition is the boolean expression we check on each iteration to check whether the loop should continue
• The final-expression is code that is always executed last of all on each iteration (usually to increment/decrement the counter)

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

While loops

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A while loop has the following general structure:

while(boolean condition) {
    //  ... code to execute on each iteration
}

The block of code within the loop will be run while the boolean condition evaluates to true. It is very easy to cause an infinite loop with a while loop as you can easily forget to ensure that the boolean condition will change at some point.

break

We can use the break keyword to exit a loop

for (let i = 0; i < 10; i++) {
    console.log(i);
    if (i > 5) {
        break;
    }
}

continue

We can use the continue keyword to stop the current iteration and skip to the next

for (let i = 0; i < 10; i++) {
    if (i % 2 === 0) {
        continue;
    }
    console.log(i);
}

Code examples

There are a set of code examples that accompany this course showing the use of JavaScript. (Right click and open in a new window/tab) if you’re viewing this on Learning Central.

• Variable Types [HTML] [Demo]
• Numerical Operators [HTML] [Demo]
• Booleans [HTML] [Demo]
• Looping [HTML] [Demo]