Control Structures in JavaScript

Conditional statements execute code only when certain conditions are met.

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1.1 if Statement

Executes a block of code if a condition evaluates to true.

Syntax

if (condition) {
  // code runs if condition is true
}

Example

let age = 20;

if (age >= 18) {
  console.log("You are eligible to vote.");
}

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1.2 if...else Statement

Executes one block if the condition is true, otherwise executes another.

let temperature = 25;

if (temperature > 30) {
  console.log("It's a hot day!");
} else {
  console.log("The weather is nice.");
}

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1.3 else if Ladder

Used when multiple conditions need to be checked in order.

let score = 85;

if (score >= 90) {
  console.log("Grade: A");
} else if (score >= 80) {
  console.log("Grade: B");
} else if (score >= 70) {
  console.log("Grade: C");
} else {
  console.log("Grade: F");
}

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The switch statement evaluates an expression and matches it against multiple values. It’s cleaner than long if...else chains when checking one variable against many values.

Syntax

switch (expression) {
  case value1:
    // code
    break;
  case value2:
    // code
    break;
  default:
    // fallback code
}

Example

let day = 3;
let dayName;

switch (day) {
  case 1:
    dayName = "Monday";
    break;
  case 2:
    dayName = "Tuesday";
    break;
  case 3:
    dayName = "Wednesday";
    break;
  case 4:
    dayName = "Thursday";
    break;
  case 5:
    dayName = "Friday";
    break;
  case 6:
    dayName = "Saturday";
    break;
  case 7:
    dayName = "Sunday";
    break;
  default:
    dayName = "Invalid day";
}

console.log(dayName); // Wednesday

Important Notes

• break prevents fall-through to the next case
• default runs if no case matches (optional but recommended)

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Loops allow you to repeat a block of code while a condition is true.

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3.1 for Loop

Best used when you know the number of iterations in advance.

Syntax

for (initialization; condition; increment) {
  // repeated code
}

Example

for (let i = 0; i < 5; i++) {
  console.log("Number:", i);
}

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3.2 while Loop

Runs as long as the condition is true. Use it when iterations are uncertain.

let count = 0;

while (count < 3) {
  console.log("Count is:", count);
  count++;
}

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3.3 do...while Loop

Executes the block at least once, even if the condition is false initially.

let num = 5;

do {
  console.log("Number is:", num);
  num++;
} while (num < 3);

📌 Output:

Number is: 5

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Control Structures in JavaScript

Structure	Best Use Case
if	Simple decision making
else if	Multiple conditions
switch	Many values for one variable
for	Known number of iterations
while	Condition-based repetition
do...while	Must run at least once

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Control structures are the backbone of JavaScript logic:

• if, else, else if → Decision making
• switch → Cleaner multi-value branching
• Loops (for, while, do...while) → Repetitive execution