StartsWith

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题目

实现StartsWith<T, U>,接收两个 string 类型参数,然后判断T是否以U开头,根据结果返回true或false

例如:

type a = StartsWith<"abc", "ac">; // expected to be false
type b = StartsWith<"abc", "ab">; // expected to be true
type c = StartsWith<"abc", "abcd">; // expected to be false

解答

type StartsWith<T extends string, U extends string> = U extends ""
  ? true
  : T extends `${U}${infer _}`
  ? true
  : false;

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take it a litter change type StartsWith<T extends string, U extends string> = T extends ${U}${infer Other}?true:false

if T extends ${U}${some} , Prove that T contains U, but U not prove that T

so: Expect<Equal<StartsWith<'abc', 'ab'>, true>>, Expect<Equal<StartsWith<'abc', 'abcd'>, false>>,

type StartsWith<
  T extends string,
  U extends string
> = T extends `${U}${string}` ? true : false;