Don’t use the types
Number, String, Boolean, Symbol or Object.1// Do
2function reverse(s: string): string;1// Don't
2function reverse(s; String): String;Don’t use generic type
T if you don’t use it! (More)1// Do
2interface Named<T> {
3 name: string;
4 value: T; // <-- added
5}1// Don't
2interface Named<T> {
3 name: string;
4}Don’t use
any. At least, use unknown.Don’t return
any for callbacks whose value will be ignored.1// Do
2// It prevents you from accidentally using the return value of x.
3function fn(x: () => void) {
4 x();
5}1// Don't
2function fn(x: () => any) {
3 x();
4}It’s always legal to provide a callback that accepts fewer args. So, don’t use optional params in callbacks.
1// Do
2interface Fetcher {
3 getObject(done: (data: unknown, elapsedTime: number) => void): void;
4}1// Don't
2interface Fetcher {
3 getObject(done: (data: unknown, elapsedTime?: number) => void): void;
4}Don’t overload callbacks.
1// Do
2declare function beforeAll(
3 action: (done: DoneFn) => void,
4 timeout?: number
5): void;1// Don't
2declare function beforeAll(action: () => void, timeout?: number): void;
3declare function beforeAll(
4 action: (done: DoneFn) => void,
5 timeout?: number
6): void;Don’t put more general overloads before more specific overloads → TS uses the first matching overload.
1// Do
2declare function fn(x: HTMLElement): number;
3declare function fn(x: unknown): unknown;1// Don't
2declare function fn(x: unknown): unknown;
3declare function fn(x: HTMLElement): number;Use trailing params instead of overloads.
1// Do
2interface Example {
3 diff(one: string, two?: string, three?: boolean): number;
4}1// Don't
2interface Example {
3 diff(one: string): number;
4 diff(one: string, two: string): number;
5 diff(one: string, two: string, three: boolean): number;
6}Don’t write overloads that differ by type in only one argument position.
1// Do
2interface Moment {
3 utcOffset(b: number | string): Moment;
4}1// Don't
2interface Moment {
3 utcOffset(b: number): Moment;
4 utcOffset(b: string): Moment;
5}To be continued…