TypeScript Functions

TypeScript provides several ways to describe how functions are called and how they relate input to output.

Describing Functions

• Function Type Expressions: Syntactically similar to arrow functions (e.g., (a: string) => void). Parameter names are required.
• Call Signatures: Used when a function has properties (defined within an object type using : instead of =>).
• Construct Signatures: Used for functions callable with new (e.g., new (s: string): SomeObject).

Generic Functions

Used to describe a correspondence between input and output types.

function firstElement<Type>(arr: Type[]): Type | undefined {
  return arr[0];
}

• Inference: TypeScript usually infers the type parameter automatically.
• Constraints: Use extends to limit the types a parameter can accept (e.g., <Type extends { length: number }>).

Optional and Rest Parameters

• Optional Parameters: Mark with ? (e.g., x?: number). They get the type type | undefined.
• Default Parameters: Provide a value (e.g., x = 10). The type is inferred from the default.
• Rest Parameters: Use ... to accept an unbounded number of arguments (e.g., ...m: number[]).
• Rest Arguments: Use spread syntax to provide arguments from an array (e.g., f(...args)). Use as const for literal tuple inference.

Function Overloads

Define multiple overload signatures followed by a single implementation signature.

• The implementation signature is not visible from the outside and must be compatible with all overloads.
• Guideline: Prefer union types over overloads when possible for simplicity.

Special Types for Functions

• void: For functions that don't return a value. Implementation can return a value (which will be ignored) if contextually typed as void.
• unknown: Safer alternative to any for arbitrary values.
• never: For functions that throw exceptions or never terminate.
• Function: Global type for all functions; calls return any (unsafe).

Other Features

• this in Functions: Declare the type of this as the first parameter (e.g., function(this: User) { ... }).
• Parameter Destructuring: Type the object after the destructuring syntax (e.g., function sum({ a, b, c }: ABC)).

References

• Source: 00_Raw/typescript-handbook.md
• typescript-moc
• typescript-generics
• typescript-objects