TypeScript adds type annotations to JavaScript. This tells the compiler what kind of data a variable should hold.

Primitive Types:

  • number - For numbers (integers, decimals, negative numbers)
  • string - For text data
  • boolean - For true/false values
  • Date - For date and time values

Array Types:

  • string[] - Array of strings
  • number[] - Array of numbers
  • boolean[] - Array of booleans

Type Safety: Once you declare a variable with a type, TypeScript prevents you from assigning values of different types:

let userId: number = 123;
userId = "456"; // ❌ Error! Can't assign string to number

let userName: string = "Sarah";
userName = 42; // ❌ Error! Can't assign number to string

Implicit Types: TypeScript can often figure out the type automatically from the value you assign:

// TypeScript infers these types automatically
let age = 25; // TypeScript knows this is number
let name = "Alex"; // TypeScript knows this is string
let isStudent = true; // TypeScript knows this is boolean

Special Types: TypeScript has some special types that don’t fit the basic categories:

null and undefined:

  • null - Explicitly represents “no value” or “empty”
  • undefined - Variable is declared but hasn’t been assigned a value

any:

  • any - Turns off TypeScript’s type checking completely
  • Use sparingly - it defeats the purpose of TypeScript
  • Useful when working with external libraries or unknown data

never:

  • never - Represents values that never occur
  • Common in functions that always throw errors or never return
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