For Loops

The most common kind of loop in any programming language is probably the for loop. This is a structure that allows us to define a clear number of times to loop through some set of instructions. The for loop can be used anytime we need to repeat a set of instructions for a specific number of iterations.

Here is an example of a for loop in JavaScript:

for(let i=0; i<10; i++) {
    console.log(`This is iteration number ${i}.`);
}

The for loop structure is not too complex, but it does require a specific syntax to make it happen. First, it identifies as a for loop with the keyword for. Then we list three statements in the top line of the loop:

1. Initializer: The first statement initializes a counter variable to a value. (This could be any variable name, but the letter i for "iteration" is often used.) This is the value where the counter will begin.
2. Exit Condition: The second statement provides a conditional assertion that is checked on each iteration of the loop. Each time the loop executes, it checks this assertion to see if it's true. If so, the loop continues. If not, the loop stops.
3. Final Expression: third statement increments the value of the counter by one. This causes the value of the counter to increase, so eventually the second statement will return false.

Here is another example of how a for loop can function:

let simpsons = ['Homer', 'Marge', 'Bart', 'Lisa', 'Maggie'];

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

Notice that in this example we are actually using the i variable (which is the counter for the for loop) as the index for which name we want to output. This code would output each of the names to the JavaScript console in the web browser.

Exercises

Please try working these exercises to practice some of the skills we've learned in this section.

let x = 2;

// for loop goes here
// A bug in the exercise plugin prevents us from showing this solution in full.

assert(x===8192, "Incorrect.");