Chapter 2: Data Types + Type System

What are Data Types?

• In JavaScript, every value has a type.
• These types define what kind of data is being stored - number, text, boolean, object, etc.

There are two categories:

• Primitive types - stored directly
• Reference / Non-Primitive types - stored as memory references

Primitive Data Types

1. String → Text
   "hello", 'Shoaib', `Shaikh`
2. Number → Any numeric value
   3, -99, 3.14
3. Boolean → True or False
   true, false
4. Undefined → Variable declared but not assigned

let x;
x // Output → undefined

5. Null → Intentional empty value

let x = null;

6. Symbol → Unique identifier (rarely used)
7. BigInt → Very large integers

123456789012345678901234567n

These are copied directly:

let a = 2;
let b = a;
b = b + 1;
b; // output → 3
a; // output → 2

Reference / Non-Primitive Data Types

1. Object → { name: "Shoaib", age: 22 }
2. Array → [10, 20, 30]
3. Function → function studentName() { }

These are not copied directly, but by reference:

let a = [10, 20, 30];
let b = a;
b.pop();
b; // As expected the output will be → [10, 20]
a; // But the output of a is also → [10, 20]

typeof Operator

• Used to check the data type of a value:

typeof "Shoaib" // "string"
typeof 99 // "number"
typeof true // "boolean"
typeof undefined // "undefined"
typeof null // "object" ← known bug
typeof [] // "object"
typeof {} // "object"
typeof function(){} // "function"

Type Coercion (Auto-Conversion)

• JavaScript auto-converts types in some operations:

"5" + 1 // "51" → number converted to string
"5" - 1 // 4 → string converted to number
true + 1 // 2
null + 1 // 1
undefined + 1 // NaN

Loose vs Strict Equality

• == compares value with type conversion
• === compares value + type (no conversion)

5 == "5" // true
5 === "5" // false

• Always prefer === for accurate comparisons.

NaN – Not a Number

typeof NaN // "number"

• Even though it means "Not a Number", NaN is actually of type number.
• This is because operations like 0 / 0 or parseInt("abc") still produce a numeric result — just an invalid one.

Truthy and Falsy Values

Falsy values:

false, 0, "", null, undefined, NaN

Everything else is truthy, including:

Truthy values:

"0", "false", [], {}, function(){}

// Example:
if ("0") {
  console.log("Runs"); // "0" is a non-empty string = truthy
}

Mindset

• JavaScript will often auto-convert types behind the scenes.
• Always stay aware of what data type you're working with.

Common Confusions

• typeof null is "object" — this is a bug.
• undefined means the variable was never assigned.
• null means you intentionally set it to "nothing".
• '5' + 1 is "51" but '5' - 1 is 4.

Practice Zone

1. Predict the output:

console.log(null + 1);
console.log("5" + 3);
console.log("5" - 3);
console.log(true + false);

2. Check types:

console.log(typeof []);
console.log(typeof null);
console.log(typeof 123n);

3. Truthy or Falsy?

console.log(Boolean(0)); // falsy
console.log(Boolean("0")); // truthy
console.log(Boolean([])); // truthy
console.log(Boolean(undefined)); // falsy

4. Write a function isEmpty(value) that checks if a given value is null, undefined, or "".
5. Compare with loose vs strict:

console.log(5 == "5"); // ?
console.log(5 === "5"); // ?