Introduction
JavaScript (JS) is the programming language of the web. HTML gives a web page its structure, CSS gives it style, and JavaScript gives it behaviour — it makes pages interactive, responds to clicks, validates forms, updates content without reloading, and much more. It is one of the three core technologies of the World Wide Web and runs inside every modern browser.
For government job and IT-officer exams (BCS, Bank, and various IT posts), JavaScript appears in the Web Technologies section. Questions are usually conceptual: What is JavaScript? Is it compiled or interpreted? What is the difference between == and ===? What are var, let, and const? What does typeof null return? This lesson builds the topic from first principles so you can answer both theory and small code-output questions with confidence.
Key fact for MCQs: JavaScript is a high-level, interpreted (JIT-compiled), dynamically-typed, single-threaded scripting language. It was created by Brendan Eich at Netscape in 1995. Its official standard is ECMAScript (ECMA-262). JavaScript is NOT the same as Java — the similar name was a marketing decision.
Learning Objectives
After studying this lesson you will be able to:
- Explain what JavaScript is, its history, and where it runs (client-side and server-side).
- Distinguish JavaScript from Java and from ECMAScript.
- Declare variables correctly using
var,let, andconstand explain scope and hoisting. - List and identify JavaScript's data types (primitive vs. reference/object).
- Use operators correctly, especially
==vs===and type coercion. - Write conditionals, loops, and functions (including arrow functions).
- Predict the output of small code snippets — the most common exam format.
- Avoid the classic mistakes examiners love to test.