Introduction to JavaScript

JavaScript is a programming language for describing operations on values. Its standard defines expressions, objects, functions, modules, promises, and built-in types. A host supplies the outside world: a browser supplies the document and user events; Node.js supplies files, processes, and servers. document.querySelector() is a browser API, while node:fs is a Node API. Neither is part of the language itself.

The language is dynamically typed: values have types, but a variable may hold values of different types over time. Functions are values too; you can pass them as arguments or return them. Objects inherit behavior through prototypes, while class offers familiar syntax over that model. Asynchronous operations commonly return promises.

const name = "Ada";
const greet = (person) => `Hello, ${person}`;
console.log(greet(name)); // Hello, Ada

This code runs in a browser console or Node.js. The console is supplied by those hosts. The template literal and arrow function are language features.

A useful mental model

1. ECMAScript defines what JavaScript code means.
2. The engine executes that code.
3. The host offers APIs for input, output, timers, and network access.
4. Your application combines language features with host APIs.

JavaScript and Java are separate languages despite the similar names. You can learn JavaScript without knowing Java. Later chapters distinguish browser behavior from Node behavior whenever they differ.

Practice: Change the string value assigned to name in the example and run it again. Call greet() with two different names, then identify which parts would still work in Node.js.

For the full language reference, see the MDN JavaScript Guide and the ECMAScript specification.

What the runtime does

An engine parses source code, checks its syntax, and executes it. Popular engines include V8, SpiderMonkey, and JavaScriptCore. A browser or Node.js embeds an engine and adds its own APIs. The same expression can therefore run in both places, while code that expects document only runs in a browser and code that imports node:fs only runs in Node.js.

This distinction helps diagnose errors. A syntax error means the source cannot be parsed. A runtime error occurs while valid code runs, for example when it reads a property from null. A logic error is valid code that produces the wrong result. Check grammar for syntax errors, inspect values and stack traces for runtime errors, and compare actual output with intended behavior for logic errors.

Where JavaScript is used

• Browser interfaces: DOM updates, forms, events, animation, and network requests.
• Servers and tools: Node.js can read files, serve HTTP, run command-line programs, and automate tasks.
• Other hosts: Desktop apps, embedded devices, and mobile frameworks can embed JavaScript engines and expose additional APIs.

JavaScript does not provide every API in every host. Check runtime documentation when an example uses fetch, process, timers, or the DOM. Also distinguish the language from a framework: React, for example, is a library used to build interfaces, not another JavaScript syntax.

When learning a feature, trace its input values, operation, result, and side effects. Change one input and predict the result before running it. This builds a mental model instead of memorizing one output.

Checkpoint: Explain why this runs in Node while document.querySelector("h1") does not. Identify the language feature and host API in const greet = (name) => "Hello, " + name;.

Written By: Shiva Srivastava