Conditions and Loops

Conditionals

Conditionals allow you to control the flow of your program based on certain conditions. JavaScript provides several ways to handle conditional logic.

if Statement

The if statement is the simplest form of conditional. It executes a block of code only if a specified condition evaluates to true.

if (condition) {
  // Code to execute if condition is true
}

Example:

let age = 18;
if (age >= 18) {
  console.log("You are an adult");
}

if...else Statement

The if...else statement allows you to specify a block of code to execute when the condition is true, and another block of code to execute when it is false.

if (condition) {
  // Code to execute if condition is true
} else {
  // Code to execute if condition is false
}

Example:

let age = 16;
if (age >= 18) {
  console.log("You are an adult");
} else {
  console.log("You are a minor");
}

else if Statement

The else if statement allows for multiple conditions to be checked in sequence. If the first condition is false, it checks the next one, and so on.

if (condition1) {
  // Code to execute if condition1 is true
} else if (condition2) {
  // Code to execute if condition2 is true
} else {
  // Code to execute if none of the conditions are true
}

Example:

let age = 20;
if (age >= 65) {
  console.log("You are a senior");
} else if (age >= 18) {
  console.log("You are an adult");
} else {
  console.log("You are a minor");
}

Ternary Operator

The ternary operator provides a shorthand for an if...else statement. It's used for simple conditional expressions.

condition ? expressionIfTrue : expressionIfFalse;

Example:

let age = 18;
let status = age >= 18 ? "Adult" : "Minor";
console.log(status); // 'Adult'

Switch Statement

A switch statement is used to execute one of many blocks of code based on the value of a variable or expression. It’s an alternative to multiple if...else if statements when checking for several possible conditions.

switch (expression) {
  case value1:
    // Code to execute if expression === value1
    break;
  case value2:
    // Code to execute if expression === value2
    break;
  default:
  // Code to execute if expression doesn't match any value
}

Example:

let day = "Monday";
switch (day) {
  case "Monday":
    console.log("Start of the week");
    break;
  case "Friday":
    console.log("End of the week");
    break;
  default:
    console.log("Midweek");
}

Loops in JavaScript

Loops allow you to repeat a block of code multiple times until a specified condition is met.

for Loop

The for loop is the most commonly used loop, ideal when you know the number of iterations beforehand.

for (initialization; condition; increment / decrement) {
  // Code to execute on each iteration
}

Example:

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

while Loop

The while loop executes the block of code as long as the specified condition evaluates to true. If the condition is false initially, the block will not execute.

while (condition) {
  // Code to execute while the condition is true
}

Example:

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

do...while Loop

The do...while loop is similar to the while loop, but it guarantees that the block of code is executed at least once before checking the condition.

do {
  // Code to execute at least once
} while (condition);

Example:

let i = 0;
do {
  console.log(i); // Outputs: 0 1 2 3 4
  i++;
} while (i < 5);

for...in Loop

The for...in loop is used to iterate over the properties of an object. It is not commonly used for arrays because the order of properties is not guaranteed.

for (let key in object) {
  // Code to execute for each property of the object
}

Example:

let person = { name: "Alice", age: 25 };
for (let key in person) {
  console.log(key + ": " + person[key]);
}
// Outputs:
// name: Alice
// age: 25

for...of Loop

The for...of loop is used to iterate over iterable objects like arrays, strings, and other collections. It provides a simpler way to loop through the values of an iterable.

for (let item of iterable) {
  // Code to execute for each item in the iterable
}

Example:

let numbers = [1, 2, 3, 4, 5];
for (let num of numbers) {
  console.log(num); // Outputs: 1 2 3 4 5
}

Important Notes on Loops and Conditionals

Short-circuiting in Logical Operators

• JavaScript uses short-circuit evaluation in logical operators (&&, ||). This means the evaluation stops as soon as the result is determined.
• && stops when a false value is encountered, and || stops when a true value is encountered.

Example:

let x = 0;
if (x && 1 / x) {
  // This code will not run because x is falsy (0).
}

Breaking and Continuing in Loops

• You can use the break statement to exit a loop early, and the continue statement to skip the current iteration and move to the next.

Example:

for (let i = 0; i < 5; i++) {
  if (i === 2) {
    continue; // Skip the iteration when i equals 2
  }
  console.log(i); // Outputs: 0 1 3 4
}

Infinite Loops

• Be cautious with loops that have conditions that never become false, as they can result in infinite loops, which can crash the browser or application.

Example:

while (true) {
  // This will run forever unless explicitly broken out of
}