Control Flow

Branching with if/else and switch, the for, while and do...while loops, leaving a loop with break and skipping a step with continue.

IT Abyss

Control flow

A program runs top to bottom, one line after another. Control-flow statements change that order: a branch chooses which piece of code to run, and a loop repeats a piece of code while a condition holds.

In JavaScript a condition does not have to be a boolean: if, while and for convert it to true or false by the truthy and falsy rules, while switch compares values strictly, like ===. Both rules are covered in detail in Coercion & Operators.

Topic map

if and else — branching on a condition, the else if chain, and the ternary operator as the short form.
switch — choosing a branch by value, strict comparison, default, and fall-through without break.
Loops — for, while, do...while: what a loop is made of and when to pick which; for...of and for...in.
break and continue — leaving a loop early, skipping a step, and labels for nested loops.

Common traps

Mistake	Consequence
`if (x = 5)` instead of `if (x === 5)`	Assignment: `x` becomes `5` and the condition is always true
A missing `break` in a `case`	Execution falls through into the following `switch` branches
`switch` on the string `'1'` with `case 1:`	No branch matches: `switch` compares strictly, without coercion
The bound `i <= arr.length`	One step too many: the last array index is `arr.length - 1`, and the extra step reads `undefined`
Forgetting to change the counter in `while`	The condition never becomes false — an infinite loop
`continue` in `while` before the counter increment	The counter stops growing — an infinite loop
`break` inside a `forEach` callback	`SyntaxError`: `forEach` cannot be stopped, use a plain loop

Interview relevance

Conditions and loops are rarely asked about directly, but they sit inside every "what does the program print" task and every live-coding task. The interviewer watches whether you step through a loop carefully: the counter's value on each iteration, where break fires, where continue jumps.

What is usually checked:

How many times the loop body runs and what the counter equals afterwards.
How while differs from do...while.
What switch does without break and how it compares values.
How break differs from continue and how to leave a nested loop.
How for...of differs from for...in.

Typical wrong answer: "continue ends the loop." In fact continue skips only the rest of the current step and the loop moves on to the next one; break is what ends the loop.

Beginner

Control Flow

Branching with if/else and switch, the for, while and do...while loops, leaving a loop with break and skipping a step with continue.

Answer it yourself — Quiz

Sections

Contents

Control flow

Topic map

if and else — branching on a condition, the else if chain, and the ternary operator as the short form.
switch — choosing a branch by value, strict comparison, default, and fall-through without break.
Loops — for, while, do...while: what a loop is made of and when to pick which; for...of and for...in.
break and continue — leaving a loop early, skipping a step, and labels for nested loops.

Common traps

Mistake	Consequence
`if (x = 5)` instead of `if (x === 5)`	Assignment: `x` becomes `5` and the condition is always true
A missing `break` in a `case`	Execution falls through into the following `switch` branches
`switch` on the string `'1'` with `case 1:`	No branch matches: `switch` compares strictly, without coercion
The bound `i <= arr.length`	One step too many: the last array index is `arr.length - 1`, and the extra step reads `undefined`
Forgetting to change the counter in `while`	The condition never becomes false — an infinite loop
`continue` in `while` before the counter increment	The counter stops growing — an infinite loop
`break` inside a `forEach` callback	`SyntaxError`: `forEach` cannot be stopped, use a plain loop

Interview relevance

What is usually checked:

How many times the loop body runs and what the counter equals afterwards.
How while differs from do...while.
What switch does without break and how it compares values.
How break differs from continue and how to leave a nested loop.
How for...of differs from for...in.

Typical wrong answer: "continue ends the loop." In fact continue skips only the rest of the current step and the loop moves on to the next one; break is what ends the loop.

Why it matters

Almost every program decides which branch to run and how many times to repeat an action. An off-by-one loop bound, a missing break in a switch or an endless while are the most common beginner bugs, and interviews catch them with "what does the program print" tasks. Once you read conditions and loops confidently, you can follow a program without running it.

Control Flow

Branching with if/else and switch, the for, while and do...while loops, leaving a loop with break and skipping a step with continue.

IT Abyss

Control flow

Topic map

if and else — branching on a condition, the else if chain, and the ternary operator as the short form.
switch — choosing a branch by value, strict comparison, default, and fall-through without break.
Loops — for, while, do...while: what a loop is made of and when to pick which; for...of and for...in.
break and continue — leaving a loop early, skipping a step, and labels for nested loops.

Common traps

Mistake	Consequence
`if (x = 5)` instead of `if (x === 5)`	Assignment: `x` becomes `5` and the condition is always true
A missing `break` in a `case`	Execution falls through into the following `switch` branches
`switch` on the string `'1'` with `case 1:`	No branch matches: `switch` compares strictly, without coercion
The bound `i <= arr.length`	One step too many: the last array index is `arr.length - 1`, and the extra step reads `undefined`
Forgetting to change the counter in `while`	The condition never becomes false — an infinite loop
`continue` in `while` before the counter increment	The counter stops growing — an infinite loop
`break` inside a `forEach` callback	`SyntaxError`: `forEach` cannot be stopped, use a plain loop

Interview relevance

What is usually checked:

How many times the loop body runs and what the counter equals afterwards.
How while differs from do...while.
What switch does without break and how it compares values.
How break differs from continue and how to leave a nested loop.
How for...of differs from for...in.

Typical wrong answer: "continue ends the loop." In fact continue skips only the rest of the current step and the loop moves on to the next one; break is what ends the loop.