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Primitive Types

In TypeScript, you annotate variables utilizing a colon syntax : type right after the variable name.

The Core Primitives​

// Number (Floats and Ints share the same type)
let price: number = 29.99;
let count: number = 100;

// String
let username: string = "Alice";

// Boolean
let isStudent: boolean = true;

If you try to reassign one of these variables to a non-matching data type, the compiler will instantly throw an error.

let score: number = 85;

// TS Error: Type 'string' is not assignable to type 'number'.
score = "Eighty-Five";

Type Inference​

TypeScript is incredibly smart. You do not need to explicitly state the type if you assign a value immediately upon creation. TS will "infer" the type natively.

// TS automatically infers that this is a Boolean!
let isOnline = false;

// TS Error: Type 'number' is not assignable to type 'boolean'.
isOnline = 15;

Should you rely on Inference? Yes! For simple primitives, relying on inference keeps your code looking much cleaner. You generally only need explicit : type syntax if you are declaring a variable without assigning its value instantly.