Function Type ↔ Type to annotate Methods & Callback Function with implicit return of any
use function type annotations to
use ": void" for functions that
use ": [never](never)" for functions that
should never complete execution
function type inference
TSC only infers Type of return values, ignores Type of param args
TS annotate the following function:
const i = 42
const logNum = (i) => console.log(i)
const logNum: (i: number) => void = (i: number) => console.log(i) const logNum = (i: number): void => console.log(i)
const logNum: (i: number) => void = (i: number) => console.log(i)
void indicates that nothing is returned
TS annotate the following function:
const add = (a, b) => a + b;
const add = (a: number, b: number): number => a + b;
TS annotate destructured function params:
const logWeather = (forecast: { date: Date; weather: string }): void =>
console.log(forecast.date, forecast.weather);
const logWeather = ({date, weather}: { date: Date; weather: string }): void => console.log(date, weather);
Typed [Rest Parameter](rest-parameter)
TS Syntax for Rest Parameter which must always be of Type Array
Function Overload
Design Pattern to reuse multiple versions of same function with various Signatures
!gotcha f: Function implies unsafe return of any
!alt use Arrow Function ()⇒void as Type for annotating functions that will not be called
Typed Callback Function
Type Annotation for Anon Function pass as arg to function
Explicit Type Annotation for return Type not required since TSC does Type Inference
Parameter Default Value applies implicit Type Inference
const toString: (num: number) ⇒ string = (num: number) ⇒ String(num)
equiv to
const toString: (num: number) => string = (num) => String(num);
equiv to
const toString = (num: number) => String(num);
callback function annotation
function stringify123(callback: (num: number) => string) {
return callback(123);
}