TypeScript Types, Inference and Functions

Primitive Types

TypeScript provides several built-in primitive types.

let username: string = "John";
let age: number = 25;
let isAdmin: boolean = false;

Type Inference

TypeScript often figures out types automatically.

let username = "John";

This process is called type inference.

Better Editor Support

Because TypeScript knows the type of a variable, editors can provide autocomplete and intelligent suggestions.

Compilation Still Happens With Errors

let age: number = "hello";

TypeScript reports an error, but JavaScript can still be generated depending on compiler settings.

The Any Type

let value: any = 10;
value = "hello";
value = true;

The any type disables type checking.

Implicit Any

let value;

When TypeScript cannot determine a type, it may infer any.

Function Parameters

function add(a: number, b: number) {
  return a + b;
}

Return Types

function add(a: number, b: number): number {
  return a + b;
}

Default Parameters

function greet(name: string = "Guest") {
  return `Hello ${name}`;
}

Void

function logMessage(message: string): void {
  console.log(message);
}

Contextual Typing

const colors = ["red", "green", "blue"];

colors.map(color => color.toUpperCase());

TypeScript infers that color is a string.

Never

function crash(message: string): never {
  throw new Error(message);
}

A function returning never never successfully completes.

Summary

Type inference, function typing, any, void, and never form the foundation of everyday TypeScript development.