TypeScript While Loop

Repeat while a test is true

A while loop checks a condition, runs a block, then checks again. As long as the condition is true, the block repeats. When the condition is false, TypeScript leaves the loop and continues with the next statement.

Something inside the loop must change the values the condition looks at. If nothing changes, the test never becomes false and the program does not stop.

A complete while program

This loop prints 1 through 5. n starts at 1. Each pass prints, then adds 1. Whenn becomes 6, the condition is false and the loop ends.

let n: number = 1;
while (n <= 5) {
  console.log(n);
  n = n + 1;
}

The three parts you manage

1. Set a starting value before the loop.
2. Write a condition that can become false.
3. Update that value inside the loop.

Forget step 3 and the condition stays true forever. Forget step 1 and the condition uses a value you never set. The for loop in the next chapter packs these three into one line. while leaves them visible, which is useful when the update is not a simple + 1.

Use let for the counter. const cannot be reassigned, so it cannot step.

do while runs the body first

A do / while loop runs the block once, then checks the condition. If the condition is true, it runs again. The body always executes at least once, even when the test starts false.

let n: number = 1;
do {
  console.log(n);
  n = n + 1;
} while (n <= 3);

Notice the semicolon after while (n <= 3). A do / while needs that semicolon. A plain while does not.

When the body should run first

Use do / while when the first pass must happen before you know whether to continue: print a menu, then ask whether to show it again. Use plain while when you might need zero passes: process input only while there is input left.

const n: number = 10;
do {
  console.log("ran once, n is " + n);
} while (n < 0);

n is 10, so n < 0 is false, but the message still prints. A plainwhile (n < 0) would print nothing.

Infinite loops

while (true) never becomes false on its own. Neither does a loop whose counter never changes. The StudyGrid editor will sit there until a time limit cuts it off. Do not submit an example that cannot finish.

A later chapter covers break, which can leave a loop early. Until then, make the condition do the stopping.

while versus for

Next: the for loop, which is the usual choice for a known number of repeats.