Primitive Types

Intro to Primitive Types
Intro to Primitive Types
Now that you know how to use console.log() and variables, let's learn about type annotations - one of TypeScript's superpowers.
You've already been using values like numbers and strings. TypeScript can infer their types automatically:
const age = 25 // TypeScript knows this is a number
const name = 'Alice' // TypeScript knows this is a string
But you can also be explicit about types using annotations:
const age: number = 25
const name: string = 'Alice'
const isActive: boolean = true
The core types you'll use most often:
  • number - All numeric values (integers, decimals, negatives)
  • string - Text data (always in quotes)
  • boolean - True or false values
  • null - Intentional absence of a value
  • undefined - Variable not yet assigned
  • bigint - Arbitrarily large integers
  • symbol - Unique identifiers

Why Types Matter

Without types, errors hide until your code runs:
// JavaScript - no error until runtime
function double(x) {
	return x * 2
}
double('hello') // Returns NaN at runtime 😱
With TypeScript, you catch the mistake immediately:
function double(x: number): number {
	return x * 2
}
double('hello') // ❌ Error: Argument of type 'string' is not assignable
TypeScript's type annotations are like documentation that the compiler can verify. They tell other developers (and future you) exactly what kind of data your code expects.

Null and Undefined

TypeScript also has two special types for representing "absence of value":
  • undefined - A variable that hasn't been assigned a value
  • null - An intentional absence of any value
Understanding the difference helps you handle edge cases gracefully.
In this exercise, you'll work with these primitive types to build confidence in TypeScript's type system.
You'll also see how booleans can come from truthy and falsy valuesβ€”an everyday JavaScript pattern for checking whether a value "exists."
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