Loops and logic

9. Loops and logic#

Loops and logic are fundamental concepts in JavaScript (JS) that allow you to perform repeated actions and make decisions in your code.

9.1. Loops#

Loops are used to repeat a block of code a number of times or until a certain condition is met. There are several types of loops in JavaScript

9.1.1. for loop#

The for loop is used when you know in advance how many times you want to execute a statement or a block of statements.

for (let i = 0; i < 5; i++) {
  console.log(`Iteration number: ${i} `);
}

9.1.2. for...of loop#

The for...of loop is used to iterate over the values of an iterable object (like an array).

let arr = [10, 20, 30];
for (let value of arr) {
  console.log(value);
}

9.1.3. for...in loop#

The for...in loop is used to iterate over the properties of an object.

let obj = { a: 1, b: 2, c: 3 };
for (let key in obj) {
  console.log(`${key} : ${obj[key]}`);
}

9.1.4. while loop#

The while loop repeats a block of code as long as a specified condition is true.

let i = 0;
while (i < 5) {
  console.log("Iteration number" + i);
  i++;
}

9.1.5. do... while loop#

The do...while loop is similar to the while loop, but it executes the block of code once before checking the condition.

let i = 0;
do {
  console.log("Iteration number" + i);
  i++;
} while (i < 5);

9.2. Logic (Conditional Statements)#

Conditional statements allow you to execute different code based on different conditions.

9.2.1. if statement#

The if statement executes a block of code if a specified condition is true.

let x = 10;
if (x > 5) {
  console.log("x is greater than 5");
}

9.2.2. if...else Statement#

The if…else statement executes one block of code if a condition is true, and another block if it is false.

let x = 10;
if (x > 15) {
  console.log("x is greater than 15");
} else {
  console.log("x is not greater than 15");
}

9.2.3. else if Statement#

The else if statement is used to specify a new condition if the first condition is false.

let x = 10;
if (x > 15) {
  console.log("x is greater than 15");
} else if (x > 5) {
  console.log("x is greater than 5 but not greater than 15");
} else {
  console.log("x is 5 or less");
}

9.2.4. switch statement#

let day = 3;
switch (day) {
  case 1:
    console.log("Monday");
    break;
  case 2:
    console.log("Tuesday");
    break;
  case 3:
    console.log("Wednesday");
    break;
  default:
    console.log("Another day");
}

9.3. Example Combining Loops and Logic#

Here’s an example that combines a loop and conditional statements:

let numbers = [1, 2, 3, 4, 5];

for (let i = 0; i < numbers.length; i++) {
  if (numbers[i] % 2 === 0) {
    console.log(numbers[i] + " is even");
  } else {
    console.log(numbers[i] + " is odd");
  }
}
Contents