Lesson 10 of 20

5 mins read

While & Do-While Loops

In Plain English

while loops repeat a block of code as long as a specified boolean condition evaluates to true. do-while loops execute the block at least once before checking the condition.

The Mental Model

"A while loop is like checking if fuel is above zero before accelerating. A do-while loop is taking a test drive first and then checking if you need more fuel."

Deep Dive: How It Works

Pre-Condition (while): Condition is evaluated before every single iteration. If initially false, the loop never runs.

Post-Condition (do-while): Code block executes once guaranteed, then tests condition.

State Mutation: Always update loop control variables inside the block to avoid freezing your runtime with infinite loops.

Syntax Blueprint

while (condition) { /* do work */ stateUpdate(); }

Repeats until condition evaluates to falsy.

Core Rules to Remember

while: Loop runs 0 or more times based on condition.

do...while: Loop runs at least 1 time regardless of condition.

Termination: Ensure the condition eventually becomes false to prevent crashes.

Common Beginner Traps & How to Fix Them

Mistake:Infinite loops caused by forgetting to increment the counter.

Why it happens: If count never changes, count < 5 remains true forever, freezing the CPU.

How to fix: Ensure every while loop modifies its terminating condition variable inside the loop body.

Pro Tip: Use while loops when the exact number of iterations is unknown in advance (e.g. streaming chunks until EOF).

Live Interactive Example

Hit Run Code to see it live

While Loop State Accumulator

JavaScript ES2024

Output Console

Click "Run Code" to view the rendered output.

How it works: Loop doubles energy each turn and terminates when energy reaches 32 (>= 20).

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Implement a countdown while loop

Initialize let count = 3;
Use a while (count > 0) loop to print "Countdown: X" and decrement count by 1 each iteration.
After the loop, print "Liftoff!".

Sandbox Output

Click "Run & Check" to test your solution.

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