Default Generic Arguments

the problem link #

just paste some code and brief description here, if wanna see the full content, please click the link above.

A lot of the same rules apply in types that apply to function argument defaults. For example, you can’t have a required argument following a default argument.

case: #

type ApiRequest = unknown
type TSConfig = unknown
type ApiRequest = unknown
type TSConfig = unknown

import { Expect, Equal } from 'type-testing';

type test_ApiRequest_explicitPost = Expect<
  Equal<ApiRequest<string, 'POST'>, { data: string; method: 'POST' }>
>;

type test_ApiRequest_implicitGet = Expect<
  Equal<ApiRequest<number>, { data: number; method: 'GET' }>
>;

type test_TSConfig_default = Expect<Equal<TSConfig, { strict: true }>>;

type test_TSConfig_true = Expect<Equal<TSConfig<{ strict: true }>, { strict: true }>>;

type test_TSConfig_false = Expect<Equal<TSConfig<{ strict: false }>, { strict: false }>>;

type test_TSConfig_boolean = Expect<Equal<TSConfig<{ strict: boolean }>, { strict: boolean }>>;
import { Expect, Equal } from 'type-testing';

type test_ApiRequest_explicitPost = Expect<
  Equal<ApiRequest<string, 'POST'>, { data: string; method: 'POST' }>
>;

type test_ApiRequest_implicitGet = Expect<
  Equal<ApiRequest<number>, { data: number; method: 'GET' }>
>;

type test_TSConfig_default = Expect<Equal<TSConfig, { strict: true }>>;

type test_TSConfig_true = Expect<Equal<TSConfig<{ strict: true }>, { strict: true }>>;

type test_TSConfig_false = Expect<Equal<TSConfig<{ strict: false }>, { strict: false }>>;

type test_TSConfig_boolean = Expect<Equal<TSConfig<{ strict: boolean }>, { strict: boolean }>>;

分析： #

• 对于ApiRequest这个类型从给出的case当中可以看出，接收两个泛型参数，对应的对象

{
  data:泛型;
  method:泛型;
}
{
  data:泛型;
  method:泛型;
}

但同时要满足Equal说明第二个泛型接受一个默认参数'GET'

所以最终答案为：

type ApiRequest<T,U = 'GET'> = {
  data: T;
  method:U;
} 
type ApiRequest<T,U = 'GET'> = {
  data: T;
  method:U;
} 

• 对于TSConfig 看得出它接收什么返回什么即可;

type TSConfig<T = { strict: true }> = T
type TSConfig<T = { strict: true }> = T

虽然综上的类型可以通过case并且提交成功，但是在实际的场景下我们还需要加上类型限制；例如对于ApiRequest,U给了默认的’GET’，例如我们需要限制U的类型只能是’GET’或者’POST’，这样才能保证类型的安全性；

type ApiRequest<T,U extends 'GET'|'POST' = 'GET'> = {
  data: T;
  method:U;
} 
type ApiRequest<T,U extends 'GET'|'POST' = 'GET'> = {
  data: T;
  method:U;
} 

这样即使 类似于

const a:ApiRequest<number,'a'> = {
  data:1,
  method:'a'
} // 这样的类型也是不允许的
const a:ApiRequest<number,'a'> = {
  data:1,
  method:'a'
} // 这样的类型也是不允许的

对于TSConfig同理

type TSConfig<T extends {strict:boolean} = { strict: true }> = T
type TSConfig<T extends {strict:boolean} = { strict: true }> = T