Variables & Types

Declare variables properly with let and const, understand JavaScript's primitive and object types, and use typeof to inspect values.

JavaScriptletconsttypestypeof

Duration

⏱ About 1.5 hours

Level

📊 Beginner

Prerequisite

🎯 Lessons 1–4

OUTCOME

Pick the right declaration for every value and predict its type

What you'll learn

1Choose between let and const (never var)
2Distinguish primitives from objects
3Use typeof to inspect a value's type
4Convert between number, string, and boolean safely

1. let, const, var

javascript

let count = 0;          // mutable
const NAME = "Alice";   // immutable binding
var legacy = true;      // legacy, do not use

count = 1;              // OK
// NAME = "Bob";         // TypeError

const by default — most variables never need to change
let when you must reassign (loop counters, conditional values)
var has function scope and hoisting quirks — avoid entirely

2. Primitive Types

Type	Example	typeof
number	42, 3.14, NaN, Infinity	"number"
string	"hello", `template`	"string"
boolean	true, false	"boolean"
undefined	undefined	"undefined"
null	null	"object" (a famous bug)
bigint	9007199254740993n	"bigint"
symbol	Symbol("id")	"symbol"

3. Objects

javascript

const user = { name: "Alice", age: 30 };
const nums = [1, 2, 3];
const now  = new Date();
typeof user;  // "object"
typeof nums;  // "object" (arrays are objects)
Array.isArray(nums);  // true

💡

Use Array.isArray() to detect arrays — typeof returns "object" for them.

4. Type Conversion

javascript

Number("42");       // 42
String(42);         // "42"
Boolean(0);         // false
Boolean("");        // false
Boolean("hello");   // true

// Coercion gotchas
"5" + 3;            // "53"  (string concatenation)
"5" - 3;            // 2     (numeric subtraction)
"5" == 5;           // true  (loose equality)
"5" === 5;          // false (strict — use this!)

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗