TypeScript Simple Types

The three core primitive types

TypeScript builds on JavaScript's primitive types by letting you annotate variables, parameters, and return values with them explicitly. The three most common simple types are string, number, and boolean, matching the equivalent JavaScript primitives.

let firstName: string = "Ada";
let age: number = 28;
let isEmployed: boolean = true;

// Reassigning with the wrong type is a compile-time error
// age = "twenty-eight"; // Error: Type 'string' is not assignable to type 'number'.

Type inference

You don't always have to write type annotations. When you initialize a variable, TypeScript automatically infers its type from the assigned value. Explicit annotations are still useful for function parameters, since there is no initial value for the compiler to infer from.

let city = "London";     // inferred as string
let population = 8900000; // inferred as number

// city = 42; // Error: Type 'number' is not assignable to type 'string'.

function describeCity(name: string, pop: number): string {
  return `${name} has a population of ${pop.toLocaleString()}.`;
}

console.log(describeCity(city, population));

Arrays and other simple type variants

Simple types can be combined into arrays using square brackets, and multiple possible types can be expressed with a union using the pipe (|) symbol. These building blocks appear constantly once you move past single values.

• string - textual data, always wrapped in quotes in code.
• number - both integers and floating point values; TypeScript does not distinguish int from float.
• boolean - only the literal values true or false.
• string[] or Array<string> - an array containing only strings.
• string | number - a union type, meaning the value can be either a string or a number.

let scores: number[] = [95, 88, 76];
let id: string | number = "abc123";
id = 42; // still valid - id can be either a string or a number

function printScore(score: number): void {
  console.log(`Score: ${score}`);
}

scores.forEach(printScore);