Support type inference for `ZodString`

[tachibanayui]

We could use Typescript template literal to narrow the type provided by z.infer<T>. This is especially useful when working with discriminated unions where part of the discriminator is dynamic. For example, the type should be ​`Hello${string}world!` | undefined instead of string | undefined:

 const zstr = z.string()
  .startsWith("Hello")
  .endsWith("world!")
  .optional();

type ZStr = z.infer<typeof zstr>; // `Hello${string}world!` | undefined

// Scenario 1: Works fine
const data: ZStr = "Hello world!"; 
zstr.parse(data);

// Scenario 2:
const data: ZStr = "I don't follow rules!"; // ts(2322): Type '"I don't follow rules!"' is not assignable to type '`Hello${string}world!`
zstr.parse(data); // Error

Here is an example of discriminated unions:

const ZCat = z.object({
    discrimiator: z.string().startsWith("cat"),
    claws: z.array(z.string().uuid()),
})
const ZBird = z.object({
    discrimiator: z.string().startsWith("bird"),
    wings: z.array(z.string().uuid()),
});

const ZAnimal = z.union([ZCat, ZBird]);
type Animal = z.infer<typeof ZAnimal>;
const catWithWings: Animal = {
    discrimiator: "cat/abyssinian",
    // Oh no! Typescript think our cat have wings is valid!
    wings: [faker.datatype.uuid()]
};

// Zod will catch the error
ZAnimal.parse(catWithWings) 

Here's an example after these changes:

// ... zod definitions above
type Animal = z.infer<typeof ZAnimal>;
const catWithWings: Animal = {
    discrimiator: "cat/abyssinian",
    // ts(2232): Type '{ discrimiator: "cat/abyssinian"; wings: string[]; }' is not assignable to type
    // '{ discrimiator: `cat${string}`; claws: string[]; } | { discrimiator: `bird${string}`; wings: string[]; }'.
    wings: [faker.datatype.uuid()],
};

While we are at it, I think it is beneficial to implement a method that checks for a substring. I will call it includes for now for its similarity with Javascript standard library.

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[colinhacks]

Thanks for the PR and cool concept, but I'm not sure about it. IMO it's not worth muddying the type definition of ZodString with a bunch of generics. It's a big hit to the readability of the type signature.

Also narrowing the type signature like this would potentially break a lot of existing code and might be unexpected.

const schema = z.object({
  name: z.string().startsWith("asdf").endsWith("qwer"),
});
type Schema = z.infer<typeof schema>;

function doStuff(_: Schema) {}

const value = { name: "asdf_qwer" };
doStuff(value);
// Argument of type '{ name: string; }' is not assignable to parameter of type '{ name: StartMidEndParts<"asdf", "", "qwer">; }'.

I think template literals are sufficiently advanced that users should stick to using z.custom if they need them.

[tachibanayui]

Thanks for the awesome feedback! For readability, I think it is convenient to see type parameters when the user calls methods such as .startsWIth(), .endsWith(), and .includes(). But I agree that it shouldn't be visible if the user doesn't use those methods. For that, we can use:

// ... Rename the old ZodString to ZodSubString. I would like to rename StartMidEndParts to SubString

type MapZSubString<T> = T extends (...args: infer P) => NoSubString ? (...args: P) => ZodString : T
type NoSubString = ZodSubString<"", "", "">
type Excludes = "startsWith" | "endsWith" | "includes"
type ZodString = {
    [TProperty in keyof NoSubString]: TProperty extends Excludes
        ? NoSubString[TProperty]
        : MapZSubString<NoSubString[TProperty]>;
} & {
    _parse(input: zod.ParseInput): zod.ParseReturnType<string>;
};

static create = (params?: RawCreateParams & { coerce?: true }): ZodString => {
  return new ZodSubString({
    checks: [],
    typeName: ZodFirstPartyTypeKind.ZodString,
    coerce: params?.coerce ?? false,
    ...processCreateParams(params),
  }) as ZodString;
};

After these changes, if the user doesn't call the method mentioned above, ZodString will be returned, otherwise ZodSubString<TStart, TMid, TEnd>

// const x: ZodString
const x = z.string().max(100)
// const y: ZodSubString<"hello", "", "">
const y = z.string().startsWith("hello")
// const yx: ZodSubString<"hello", "", "">
const yx = z.string().startsWith("hello").max(100);

For type narrowing that might cause issues, I think the input should be validated before giving it to the function doStuff(...). And fortunately schema.parse() happens to convert unknown to our desired type:

const value = schema.parse({ name: "asdf_qwer" });
doStuff(value);

In case the user absolutely wants to use the input without validation, they can use the as keyword:

const value = { name: "asdf_qwer" } as Schema;
doStuff(value);

People use zod because it is meant to be a pleasant way to validate data, but writing z.custom<`prefix${string}`&`${string}suffix`>((val) => z.string().startsWith("prefix").endsWith("suffix").parse(val)) kinda defeat the purpose. But that is my own opinion, please let me know what you think. If you agree with me, let me know, and I will implement the changes mentioned above.

EDIT: I entirely forgot about existing code bases that potentially use the pattern you described. To avoid breaking changes, we could create methods specifically for templated literal: startsWithLiteral(), endsWithLiteral(), and includesLiteral() or add a 3rd argument exact?: true to avoid breaking changes thanks @santosmarco-caribou. However, I wish these methods be the default in the next major release.

[igalklebanov]

startsWithLiteral(), endsWithLiteral(), and includesLiteral() or add a 3rd argument exact?: true

I strongly believe #1786 can be a great alternative to these (they add complexity, will confuse people, and will break "what you write is what you get" - I used string() so I probably want the type to be string in the end).

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.