Control Flow: if/else, switch, and Loops

Control flow determines the order in which statements are executed. By using conditionals and loops, you can make your programs react to different inputs and repeat tasks efficiently.

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if / else if / else

The most fundamental conditional statement:

const temperature = 28;

if (temperature > 30) {
  console.log("It's hot!");
} else if (temperature > 20) {
  console.log("It's warm.");
} else if (temperature > 10) {
  console.log("It's cool.");
} else {
  console.log("It's cold!");
}
// Output: "It's warm."

Rules

• The condition inside if() is converted to a boolean.
• Only the first branch whose condition is truthy will execute.
• else if and else are optional.

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Truthy and Falsy Values

JavaScript converts any value to a boolean when needed. Values that convert to false are called falsy:

Falsy Values
false
0, -0, 0n
"" (empty string)
null
undefined
NaN

Everything else is truthy — including "0", "false", empty arrays [], and empty objects {}.

if ("hello") {
  console.log("Truthy!");   // this runs
}

if ("") {
  console.log("This never runs");
}

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The Ternary Operator

A compact alternative to if/else for simple assignments:

const age = 20;
const status = age >= 18 ? "adult" : "minor";
// "adult"

Syntax: condition ? valueIfTrue : valueIfFalse

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switch Statements

Use switch when comparing one value against many possible matches:

const day = "Monday";

switch (day) {
  case "Monday":
  case "Tuesday":
  case "Wednesday":
  case "Thursday":
  case "Friday":
    console.log("Weekday");
    break;
  case "Saturday":
  case "Sunday":
    console.log("Weekend");
    break;
  default:
    console.log("Unknown day");
}

Key Points

• Each case uses strict equality (===).
• Without break, execution falls through to the next case.
• Always include a default case for unexpected values.

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for Loop

The classic loop with an initialiser, condition, and update expression:

for (let i = 0; i < 5; i++) {
  console.log(i);
}
// 0, 1, 2, 3, 4

Anatomy of a for Loop

for (initialiser; condition; update) {
  // body
}

1. Initialiser runs once before the loop starts.
2. Condition is checked before each iteration — if false, the loop ends.
3. Update runs after each iteration.

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while Loop

Use while when you do not know how many iterations are needed:

let count = 0;
while (count < 3) {
  console.log(count);
  count++;
}
// 0, 1, 2

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do...while Loop

Guarantees the body executes at least once because the condition is checked after the first iteration: