Do–while loop in Javascript - Step-by-Step Execution

Concept Flow - Do–while loop

Start

↓

Execute loop body

↓

Check condition

Yes| No↓

Repeat

The do–while loop runs the code inside the loop first, then checks the condition. If true, it repeats; if false, it stops.

Execution Sample

Javascript

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

This code prints numbers 1 to 3 using a do–while loop.

Execution Table

Step	count before body	Action	count after body	Condition (count <= 3)	Loop continues?
1	1	Print 1, increment count to 2	2	2 <= 3	Yes
2	2	Print 2, increment count to 3	3	3 <= 3	Yes
3	3	Print 3, increment count to 4	4	4 <= 3	No

💡 At step 3, count becomes 4, condition 4 <= 3 is false, loop stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	1	2	3	4	4

Key Moments - 3 Insights

Why does the loop body run even when the condition is false at the start?

Because the do–while loop runs the body first before checking the condition, as shown in step 1 of the execution_table.

When does the condition get checked in a do–while loop?

The condition is checked after the loop body runs, as seen in the 'Condition' column after each action in the execution_table.

What happens if the condition is false after the first iteration?

The loop stops after completing the first iteration, because the condition check fails, shown in step 3 where loop continues is 'No'.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of count before the second loop body execution?

A 1

B 2

C 3

D 4

💡 Hint

Check the 'count before body' column at step 2 in the execution_table.

At which step does the loop condition become false?

A Step 1

B Step 2

C Step 3

D The condition never becomes false

💡 Hint

Look at the 'Condition' and 'Loop continues?' columns in the execution_table.

If the initial count was 4, how many times would the loop body run?

A 1 time

B 0 times

C 3 times

D Infinite times

💡 Hint

Remember do–while runs the body once before checking condition; see key_moments about first iteration.

Concept Snapshot

do {
  // code to run
} while (condition);

- Runs loop body first, then checks condition.
- Repeats if condition is true.
- Always runs at least once.

Full Transcript

A do–while loop in JavaScript runs the code inside the loop first, then checks the condition. If the condition is true, it repeats the loop. This means the loop body always runs at least once, even if the condition is false at the start. For example, if count starts at 1, the loop prints count and increments it, then checks if count is less than or equal to 3. It repeats until count becomes 4, then stops. This is different from a while loop that checks the condition before running the body. The execution table shows each step: the value of count before running the body, the action taken, the new count, the condition check, and whether the loop continues. Beginners often wonder why the loop runs once even if the condition is false initially; this is because the condition is checked after the body runs. If the initial count was 4, the loop would still run once before stopping. This behavior makes do–while useful when you want the loop to run at least once no matter what.