StartsWith

约 219 字小于 1 分钟

2022-12-01

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

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

解题思路

略。

答案

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

验证

type cases
 = [
  Expect
<Equal
<StartsWith
<'abc', 'ac'>, false>>,
  Expect
<Equal
<StartsWith
<'abc', 'ab'>, true>>,
  Expect
<Equal
<StartsWith
<'abc', 'abc'>, true>>,
  Expect
<Equal
<StartsWith
<'abc', 'abcd'>, false>>,
  Expect
<Equal
<StartsWith
<'abc', ''>, true>>,
  Expect
<Equal
<StartsWith
<'abc', ' '>, false>>,
  Expect
<Equal
<StartsWith
<'', ''>, true>>,
]

参考

泛型 Generics
泛型约束 Generics constraints
条件类型 Conditional Types
模板字面量类型 Template Literal Types