打字稿在类型级别匹配第一个大写字母

Question

我想将字符串文字从camelCase转换为snake_case，例如：

type CamelCaseStr = "helloWorldAgain"
type _ = ToSnakeCase<CamelCaseStr> // "hello_world_again"

这是我的解决方案：

type str = "helloWorldAgain";
type UpperCollection = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
type Chars<T extends string> = T extends `${infer Char}${infer Rest}` ? Char | Chars<Rest> : never;
type UpperLetters = Chars<UpperCollection>;

type ToSnakeCase<T extends string> = 
T extends `${infer LowerStr}${UpperLetters}${infer AfterFirstBigLetter}` ? 
T extends `${LowerStr}${infer WithFirstBigLetter}` ? 
`${LowerStr}_${ToSnakeCase<Uncapitalize<WithFirstBigLetter>>}` : 
LowerStr : 
T;
type _ = ToSnakeCase<str>; 
// type _ = "helloWorld_again" | "helloWorld_world_again" | "hello_again" | "hello_world_again"

我尝试提取

LowerStr

，直到第一次遇到大写字母，因此提取第一个大写的str，然后

Uncapitalize

它并再次使用

ToSnakeCase

。然而结果是错误的，所以我写了一些测试代码。

type TestToSnakeCase<T extends string> = 
T extends `${infer LowerStr}${UpperLetters}${infer AfterFirstBigLetter}` ? 
T extends `${LowerStr}${infer WithFirstBigLetter}` ? 
[LowerStr, WithFirstBigLetter] : 
never
: T;
type __ = TestToSnakeCase<str>; 
// type __ = ["helloWorld" | "hello", "Again" | "WorldAgain"];

问题似乎已经减少到消除歧视的地步。我搜索并发现这个链接很有用，我再次尝试了。

type ShutDiscrimination<T extends string> = 
[T] extends [`${infer LowerStr}${UpperLetters}${infer AfterFirstBigLetter}`] ? 
[T] extends [`${LowerStr}${infer WithFirstBigLetter}`] ? 
`${LowerStr}_${ShutDiscrimination<Uncapitalize<WithFirstBigLetter>>}` : 
LowerStr : 
T;
type ___ = ShutDiscrimination<str>;
// type ___ = "helloWorld_again" | "helloWorld_worldAgain" | "hello_again" | "hello_worldAgain"

为了解决这个问题我错过了什么关键的事情吗？我希望我能很好地表达我的想法，格式化相当麻烦:)

游乐场链接

Answer 1

代码的主要问题是您直接在推断的条件子句中使用

UpperLetters

。由于

UpperLetters

只能是单个字符，因此您应该推断该字符，然后检查它是否为

extends

UpperLetters

。这将使您看到的联合变平。

type UpperLetter = "a" | "b";
type Parse<S extends string> = S extends
  `${infer $Start}${infer $UpperLetter}${infer $Rest}`
  ? $UpperLetter extends UpperLetter ? true : false
  : false;

无论如何，我自己尝试了这个问题，没有使用静态

UpperLetter

联合，通过检查

Capitalize<S>

是否等于

S

。在这里：

type CamelToSnakeCase<S extends string, $Acc extends string = ""> =
  //
  S extends `${infer $Ch}${infer $Rest}`
    ? S extends Capitalize<S>
      ? CamelToSnakeCase<$Rest, `${$Acc}_${Lowercase<$Ch>}`>
    : CamelToSnakeCase<$Rest, `${$Acc}${$Ch}`>
    : $Acc;

type foo = CamelToSnakeCase<"helloAwesomeWorld">; // "hello_awesome_world"
type foo2 = CamelToSnakeCase<"">; // ""
type foo3 = CamelToSnakeCase<"bruhSOCool">; // "bruh_s_o_cool"

type SnakeToCamelCase<S extends string, $Acc extends string = ""> =
  //
  S extends `${infer $Ch}${infer $Rest}`
    ? $Ch extends "_"
      ? $Rest extends `${infer $Ch}${infer $Rest}`
        ? SnakeToCamelCase<$Rest, `${$Acc}${Capitalize<$Ch>}`>
      : ""
    : SnakeToCamelCase<$Rest, `${$Acc}${$Ch}`>
    : $Acc;

type asdf = SnakeToCamelCase<"hello_awesome_world">; // "helloAwesomeWorld"
type asdf2 = SnakeToCamelCase<"bruh_s_o_cool">; // "bruhSOCool"
type asdf3 = SnakeToCamelCase<"">; // ""
type asdf4 = SnakeToCamelCase<"so_cool_bro">; // "soCoolBro"

TypeScript Playground 链接

编辑：修复您的代码的不是工会。事实上，当我做

${infer $Ch}${infer $Rest}

时，我推断出下一个字母，然后通过

Capitalize

检查它是否是大写。您正在执行

${infer LowerStr}${UpperLetters}${infer AfterFirstBigLetter}

计算

UpperLetters

中每个分支的字符串值，这会导致

LowerStr

和

AfterFirstBigLetter

出现多种类型的问题。这是一个例子：

type UpperLetter = "A" | "B" | "C" | "D" // ..

// This is going to go through each computed type in the extends clause and come up with multiple 
// answers in the union for after the various characters.

type bad = "ABCD" extends `${string}${UpperLetter}${infer $After}` ? $After : never; // "" | "D" | "BCD" | "CD"

// Now, let's fix the code by writing an infer character to force the extends clause to only have one possible combination.
// This can be done by replacing `UpperLetter` with `infer $Ch` because any `string` / `infer $Ch` and removing the leading
// `${string}` part.

type fixed = "ABCD" extends `${infer $Ch}${infer $After}` ? $Ch extends UpperLetter ? $After : never : never; // "BCD"

TypeScript Playground 链接

Answer 2

这是我的解决方案，灵感来自@sno2

我关注的是只有一种方法可以将“目标字符”与递归进行比较，没有

Alphabet Collection

。

export type CamelCaseToSnakeCase<
  T extends string,
  Joiner extends '' | '_' = ''
> = T extends `${infer Character}${infer Rest}`
  ? Character extends Uppercase<Character>
    ? `${Joiner}${Lowercase<Character>}${CamelCaseToSnakeCase<Rest, '_'>}`
    : `${Character}${CamelCaseToSnakeCase<Rest, '_'>}`
  : '';