JavaScript Essentials for Node

The core JavaScript you need before you build with Node.js

Big Idea: Node.js lets you run JavaScript on your computer, outside the browser. Before you build servers and tools, you need a solid grip on the building blocks of the language: values, variables, functions, objects, and arrays. Master these and the rest of Node feels easy.

By the end you will be able to:

declare variables with let and const and know which to pick,
name the primitive types and check them with typeof,
build text with template strings,
write functions and arrow functions,
store data in objects and arrays and read it back.

1. How Node runs your code

You write JavaScript in a file that ends in .js. Then you ask Node to run it from your terminal:

node app.js

Node reads your file top to bottom and does exactly what it says. Anything you console.log(...) is printed in the terminal.

You write JavaScript in a file, Node runs it line by line, and console.log prints the result.

2. Variables: let and const

A variable is a labelled box that holds a value. In modern JavaScript you make one with let or const.

let — use it when the value will change later (a score, a counter).
const — use it when the value should stay the same (a name, a setting). This is the safer default.

let score = 0;        // can change later
const name = 'Ada';   // stays the same

score = 10;           // OK, let can be reassigned
console.log(score);   // 10
console.log(name);    // Ada

// name = 'Bob';      // TypeError: Assignment to constant variable.

Analogy: Think of a variable as a labelled jar. let is a jar you can empty and refill. const is a jar with the lid glued on — you cannot swap what it points to. Reach for const first, and switch to let only when you truly need to reassign.

JavaScript Essentials for Node