Strongly Typed Yup Schema in TypeScript

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This weekend I have been exploring conditional types in TypeScript to use with Firestore. These take the basic form of T extends U ? X : Y and are used to create a new type based on the type of T. I was curious if I could use this to create a strongly typed Yup schema.

The basic idea is to create a generic function that takes a Yup schema and a type. The function will return a Yup schema that is strongly typed to the type. The function will use a conditional type to determine the type of the schema and return the appropriate schema.

import * as yup from "yup";

export type ConditionalSchema<T> = T extends string
? yup.StringSchema
: T extends number
? yup.NumberSchema
: T extends boolean
? yup.BooleanSchema
: T extends Record<any, any>
? yup.AnyObjectSchema
: T extends Array<any>
? yup.ArraySchema<any, any>
: yup.AnySchema;

export type Shape<Fields> = {
[Key in keyof Fields]: ConditionalSchema<Fields[Key]>;

With those two generic types, we can create a strongly typed Yup schema. These can obviously be extended to include more types and better handle arrays and objects.

Some example usage that keeps the TypeScript compiler happy:

interface Foo {
stringField: string;
booleanField: boolean;

stringField: yup.string().default(""),
booleanField: yup.boolean().default(false),

And it works! 🎉


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