Loops, break, and continue

Why use loops?

A loop repeats an action. It is useful whenever we want the same statements to run more than once. Printing "ong" a single time is simple, but copying the same statement ten times is inefficient:

console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");
console.log("ong");

A loop expresses that repetition directly. This is the original article's example:

for(int i = 0; i < 10; i++){
	console.log("ong");
}

Editorial correction: JavaScript does not declare variables with int. In the example above, change int i = 0 to let i = 0. The original code is retained to make the correction explicit.

The structure of for

A for loop has three parts separated by semicolons:

1. An initialization, such as let i = 0.
2. A condition that determines whether the next iteration runs.
3. An update expression, such as i++, that runs after each iteration.

In this example, i starts at zero and increases by one each time. The loop runs while i < 10 and stops when i reaches ten, printing "ong" ten times. It does not wait until i exceeds ten.

while and do...while

A while loop puts its condition in parentheses and stops when that condition is false. The original example initializes a to zero and repeatedly writes "ong", incrementing a while the stated condition allows it.

Open the original while example on CodePen.

A do...while looks similar, but the condition appears after the body. This changes the order: while checks first and then runs the body; do...while runs the body first and checks afterward. As a result, the body of a do...while runs at least once.

Open the original do...while example on CodePen.

break and continue

You may remember break from the switch lesson. In a loop, it exits the loop entirely.

continue does something different: it skips the remainder of the current iteration. For example, in a loop that counts upward toward ten, encountering break when i is four leaves the loop. Encountering continue skips to the loop's update and then the next iteration, where i becomes five.

The original closing sentence previewed variable scope; the next published article in this archive introduces functions.