For Loops in JavaScript (OCR A Level Computer Science): Revision Note

For Loops in JavaScript

• A for loop is a control flow statement that allows a block of code to repeatedly execute for a specified number of iterations

• It provides a concise and structured way to perform repetitive tasks.

Syntax of a for loop

• The syntax of a for loop consists of three main parts:

for (initialisation; condition; increment/decrement) {
  // Code to be executed in each iteration
}

1. Initialisation: The initialisation is executed only once at the beginning of the loop. It is used to initialise a counter variable that controls the loop's execution

2. Condition: The condition is evaluated before each iteration. If the condition evaluates to true, the loop continues executing the code block. If the condition evaluates to false, the loop terminates

3. Increment/Decrement: The increment or decrement statement is executed at the end of each iteration, updating the counter variable to control the loop's progress

Example 1: Counting from 1 to 5

Example 2: Iterating over an array

'For in' loops in JavaScript

• The for in the loop iterates through the items in a data structure like a list or array

const fruits = ['apple', 'banana', 'orange', 'grape'];

for (let index in fruits) {
  console.log('Index: ' + index + ', Value: ' + fruits[index]);
}

• The list fruits contains four items: 'apple', 'banana', 'orange', and 'grape'

• The for...in the loop iterates over each index of the fruits list

• In each iteration, the index the variable is assigned the current index value

• Inside the loop, we use fruits[index] to access the value associated with the current index

• The loop executes the code block, which outputs the index and value of each item in the list to the browser