Syntax
type Name = TypeDefinition;Examples
Aliasing Object Shapes and Unions
The two most common uses of type aliases.
type Point = {
x: number;
y: number;
};
type Status = "pending" | "active" | "completed"; // union of literal types
function updateStatus(status: Status) {
console.log(`Status is now: ${status}`);
}
updateStatus("active");
// updateStatus("done"); // Error - "done" is not one of the allowed literalsAliasing Function Signatures and Tuples
Type aliases work for more than just objects.
type MathFn = (a: number, b: number) => number;
const add: MathFn = (a, b) => a + b;
const multiply: MathFn = (a, b) => a * b;
type Coordinate = [number, number]; // a tuple alias
const origin: Coordinate = [0, 0];Best practices
- Use type aliases for unions, tuples, and function signatures - things interface cannot directly express
- Give type aliases clear, descriptive names (Status, not T1) so error messages and autocomplete stay meaningful
- For plain object shapes, either type or interface works - pick one convention and stay consistent within a project
- Reuse a type alias across multiple functions/variables rather than repeating the same inline type definition
At a glance
- Purpose
- Static types for JavaScript
- File extension
- .ts · .tsx
- Runs in
- Compiled to JavaScript
- Usually used with
- JavaScript and its ecosystem
Specifications & further reading
Related TypeScript documentation
Basic Types
TypeScript extends JavaScript with static types, letting the compiler catch type errors before code ever runs. Beyond the familiar string, number, and boolean, TypeScript adds any (disables checking entirely), unknown (a safer any that requires narrowing), void (a function returning nothing), and never (a value that can never occur, like a function that always throws).Union and Intersection Types
A union type (A | B) means a value can be either type A or type B. An intersection type (A & B) means a value must satisfy both A and B simultaneously, combining their members into one type. Unions are common for values with a few valid states; intersections are common for combining smaller, focused types into a larger one.Literal Types
Literal types narrow a type down to one specific, exact value rather than a general category - "success" instead of string, or 200 instead of number. They are most useful combined with union types, letting you precisely constrain a value to a specific set of allowed options, catching typos and invalid values at compile time.Type Inference
TypeScript can automatically determine a value's type from its initializer, without an explicit annotation - this is type inference. It reduces boilerplate significantly while still providing full type safety. Understanding what TypeScript infers (and when it needs help) is key to writing idiomatic, uncluttered TypeScript.
TypeScript extends JavaScript with static types, letting the compiler catch type errors before code ever runs. Beyond the familiar string, number, and boolean, TypeScript adds any (disables checking entirely), unknown (a safer any that requires narrowing), void (a function returning nothing), and never (a value that can never occur, like a function that always throws).Union and Intersection Types
A union type (A | B) means a value can be either type A or type B. An intersection type (A & B) means a value must satisfy both A and B simultaneously, combining their members into one type. Unions are common for values with a few valid states; intersections are common for combining smaller, focused types into a larger one.Literal Types
Literal types narrow a type down to one specific, exact value rather than a general category - "success" instead of string, or 200 instead of number. They are most useful combined with union types, letting you precisely constrain a value to a specific set of allowed options, catching typos and invalid values at compile time.Type Inference
TypeScript can automatically determine a value's type from its initializer, without an explicit annotation - this is type inference. It reduces boilerplate significantly while still providing full type safety. Understanding what TypeScript infers (and when it needs help) is key to writing idiomatic, uncluttered TypeScript.