CS Notes

JavaScript Syntax

3 min read

Purpose And Data Types

JavaScript adds dynamic functionality like form validation or interactive maps

  • Integer - whole numbers. Example: let count = 5;
  • Real - decimal numbers. Example: let price = 10.99;
  • String - text. Example: let name = "Alice";
  • Boolean - true or false. Example: let isFound = false;

Variables And Constants

  • Variables (let) - can be changed during execution. Example: let score = 0; score = 10;
  • Constants (const) - remain the same. Example: const TAX_RATE = 0.2;

Arithmetic Operators

Used for mathematical calculations

  • Basic - +, -, *, /
  • Modulus (%) - returns remainder. Example: 10 % 3 is 1
  • Exponentiation (**) - power of. Example: 2 ** 3 is 8
  • Increment/Decrement - ++, --. Example: x++ adds 1 to x

Comparison And Logical Operators

  • Equal Value (==) - Example: 5 == "5" is true
  • Strict Equal (===) - checks value and type. Example: 5 === "5" is false
  • Logic - && (AND), || (OR), ! (NOT). Example: (x > 0 && x < 10)

Strings

  • Length - .length. Example: "Hello".length is 5
  • Substring - .substring(start, end). Example: "Apple".substring(0, 3) is "App"

Selection And Decision Making

  • If Statements - choose code to run if a condition is true
    if (age >= 18) { console.log("Adult"); } else { console.log("Minor"); }
  • Switch Case - useful for comparing a single expression against multiple possible values
  • break - prevents “fall-through” (executing the next case automatically)
  • default - runs if no matches are found Example:
    switch (grade) {
      case 'A': console.log("Great"); break;
      case 'B': console.log("Good"); break;
      default: console.log("Keep trying");
    }

Iteration And Loops

  • For Loops - used when the number of iterations is known
  • Standard - for (initialisation; condition; increment)
  • Example: for (let i = 0; i < 5; i++) { console.log(i); }
  • For…in - iterates over the indices of an array
  • Example: for (let x in fruits) { console.log(fruits[x]); }
  • While And Do While Loops - used when the number of iterations is unknown in advance
  • while - checks the condition before running the code. (May run 0 times)
  • Example: while (password != "secret") { password = prompt("Enter pass"); }
  • do while - checks the condition after running the code. (Always runs at least once)
  • Example: do { roll = Math.random(); } while (roll < 0.5);

Nested Statements

You can place one construct inside another (e.g., an if inside a for loop)

  • Tip - to find the total iterations of a nested loop, multiply the outer loop count by the inner loop count
  • Example:
    for (let i = 0; i < 3; i++) { // Outer
      for (let j = 0; j < 5; j++) { // Inner runs 15 times total (3 * 5)
        console.log(i, j);
      }
    }

Functions And Procedures

Blocks of code that can be reused throughout a programme

  • Function - performs a task and returns a value using the return keyword
  • Example: function add(a, b) { return a + b; }
  • Procedure - performs a task but does not return a value
  • Example: function greet() { console.log("Hello!"); }

Producing Output

  • innerHTML - modifies content within an element
  • Example: document.getElementById("msg").innerHTML = "Hello";
  • document.write() - writes directly to the document
  • Example: document.write("Goodbye!");
  • alert() - displays a pop-up box
  • Example: alert("Warning!");

Examiner Tip

If you are asked to complete a JavaScript function in the exam, always check if you need to use the return keyword to pass the result back to the programme!

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