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# Stop typing twice 🙅‍♂️ A lot of projects use JSON schemas for runtime data validation along with TypeScript for static type checking. Their code may look like this: ```typescript const dogSchema = { type: "object", properties: { name: { type: "string" }, age: { type: "integer" }, hobbies: { type: "array", items: { type: "string" } }, favoriteFood: { enum: ["pizza", "taco", "fries"] }, }, required: ["name", "age"], }; type Dog = { name: string; age: number; hobbies?: string[]; favoriteFood?: "pizza" | "taco" | "fries"; }; ``` Both objects carry similar if not exactly the same information. This is a code duplication that can annoy developers and introduce bugs if not properly maintained. That's when `json-schema-to-ts` comes to the rescue 💪 ## FromSchema The `FromSchema` method lets you infer TS types directly from JSON schemas: ```typescript import { FromSchema } from "json-schema-to-ts"; const dogSchema = { type: "object", properties: { name: { type: "string" }, age: { type: "integer" }, hobbies: { type: "array", items: { type: "string" } }, favoriteFood: { enum: ["pizza", "taco", "fries"] }, }, required: ["name", "age"], } as const; type Dog = FromSchema; // => Will infer the same type as above ``` Schemas can even be nested, as long as you don't forget the `as const` statement: ```typescript const catSchema = { ... } as const; const petSchema = { anyOf: [dogSchema, catSchema], } as const; type Pet = FromSchema; // => Will work 🙌 ``` The `as const` statement is used so that TypeScript takes the schema definition to the word (e.g. _true_ is interpreted as the _true_ constant and not widened as _boolean_). It is pure TypeScript and has zero impact on the compiled code. If you don't mind impacting the compiled code, you can use the `asConst` util, which simply returns the schema while narrowing its inferred type. ```typescript import { asConst } from "json-schema-to-ts"; const dogSchema = asConst({ type: "object", ... }); type Dog = FromSchema; // => Will work as well 🙌 ``` Since TS 4.9, you can also use the `satisfies` operator to benefit from type-checking and autocompletion: ```typescript import type { JSONSchema } from "json-schema-to-ts"; const dogSchema = { // Type-checked and autocompleted 🙌 type: "object" ... } as const satisfies JSONSchema type Dog = FromSchema // => Still work 🙌 ``` You can also use this with JSDocs by wrapping your schema in `/** @type {const} @satisfies {import('json-schema-to-ts').JSONSchema} */ (...)` like: ``` const dogSchema = /** @type {const} @satisfies {import('json-schema-to-ts').JSONSchema} */ ({ // Type-checked and autocompleted 🙌 type: "object" ... }) /** @type {import('json-schema-to-ts').FromSchema} */ const dog = { ... } ``` ## Why use `json-schema-to-ts`? If you're looking for runtime validation with added types, libraries like [yup](https://github.com/jquense/yup), [zod](https://github.com/vriad/zod) or [runtypes](https://github.com/pelotom/runtypes) may suit your needs while being easier to use! On the other hand, JSON schemas have the benefit of being widely used, more versatile and reusable (swaggers, APIaaS...). If you prefer to stick to them and can define your schemas in TS instead of JSON (importing JSONs `as const` is not available yet), then `json-schema-to-ts` is made for you: - ✅ **Schema validation** `FromSchema` raises TS errors on invalid schemas, based on [DefinitelyTyped](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/json-schema)'s definitions - ✨ **No impact on compiled code**: `json-schema-to-ts` only operates in type space. And after all, what's lighter than a dev-dependency? - 🍸 **DRYness**: Less code means less embarrassing typos - 🤝 **Real-time consistency**: See that `string` that you used instead of an `enum`? Or this `additionalProperties` you confused with `additionalItems`? Or forgot entirely? Well, `json-schema-to-ts` does! - 🔧 **Reliability**: `FromSchema` is extensively tested against [AJV](https://github.com/ajv-validator/ajv), and covers all the use cases that can be handled by TS for now\* - 🏋️‍♂️ **Help on complex schemas**: Get complex schemas right first time with instantaneous typing feedbacks! For instance, it's not obvious the following schema can never be validated: ```typescript const addressSchema = { type: "object", allOf: [ { properties: { street: { type: "string" }, city: { type: "string" }, state: { type: "string" }, }, required: ["street", "city", "state"], }, { properties: { type: { enum: ["residential", "business"] }, }, }, ], additionalProperties: false, } as const; ``` But it is with `FromSchema`! ```typescript type Address = FromSchema; // => never 🙌 ``` > \*If `json-schema-to-ts` misses one of your use case, feel free to [open an issue](https://github.com/ThomasAribart/json-schema-to-ts/issues) 🤗 ## Table of content - [Installation](#installation) - [Use cases](#use-cases) - [Const](#const) - [Enums](#enums) - [Primitive types](#primitive-types) - [Nullable](#nullable) - [Arrays](#arrays) - [Tuples](#tuples) - [Objects](#objects) - [Combining schemas](#combining-schemas) - [AnyOf](#anyof) - [AllOf](#allof) - [OneOf](#oneof) - [Not](#not) - [If/Then/Else](#ifthenelse) - [Definitions](#definitions) - [References](#references) - [Deserialization](#deserialization) - [Extensions](#extensions) - [Typeguards](#typeguards) - [Validators](#validators) - [Compilers](#compilers) - [FAQ](#frequently-asked-questions) ## Installation ```bash # npm npm install --save-dev json-schema-to-ts # yarn yarn add --dev json-schema-to-ts ``` > `json-schema-to-ts` requires TypeScript 4.3+. Using `strict` mode is required, as well as (apparently) turning off [`noStrictGenericChecks`](https://www.typescriptlang.org/tsconfig#noStrictGenericChecks). ## Use cases ### Const ```typescript const fooSchema = { const: "foo", } as const; type Foo = FromSchema; // => "foo" ``` ### Enums ```typescript const enumSchema = { enum: [true, 42, { foo: "bar" }], } as const; type Enum = FromSchema; // => true | 42 | { foo: "bar"} ``` You can also go full circle with typescript `enums`. ```typescript enum Food { Pizza = "pizza", Taco = "taco", Fries = "fries", } const enumSchema = { enum: Object.values(Food), } as const; type Enum = FromSchema; // => Food ``` ### Primitive types ```typescript const primitiveTypeSchema = { type: "null", // "boolean", "string", "integer", "number" } as const; type PrimitiveType = FromSchema; // => null, boolean, string or number ``` ```typescript const primitiveTypesSchema = { type: ["null", "string"], } as const; type PrimitiveTypes = FromSchema; // => null | string ``` > For more complex types, refinment keywords like `required` or `additionalItems` will apply 🙌 ### Nullable ```typescript const nullableSchema = { type: "string", nullable: true, } as const; type Nullable = FromSchema; // => string | null ``` ### Arrays ```typescript const arraySchema = { type: "array", items: { type: "string" }, } as const; type Array = FromSchema; // => string[] ``` ### Tuples ```typescript const tupleSchema = { type: "array", items: [{ type: "boolean" }, { type: "string" }], } as const; type Tuple = FromSchema; // => [] | [boolean] | [boolean, string] | [boolean, string, ...unknown[]] ``` `FromSchema` supports the `additionalItems` keyword: ```typescript const tupleSchema = { type: "array", items: [{ type: "boolean" }, { type: "string" }], additionalItems: false, } as const; type Tuple = FromSchema; // => [] | [boolean] | [boolean, string] ``` ```typescript const tupleSchema = { type: "array", items: [{ type: "boolean" }, { type: "string" }], additionalItems: { type: "number" }, } as const; type Tuple = FromSchema; // => [] | [boolean] | [boolean, string] | [boolean, string, ...number[]] ``` ...as well as the `minItems` and `maxItems` keywords: ```typescript const tupleSchema = { type: "array", items: [{ type: "boolean" }, { type: "string" }], minItems: 1, maxItems: 2, } as const; type Tuple = FromSchema; // => [boolean] | [boolean, string] ``` > Additional items will only work if Typescript's `strictNullChecks` option is activated ### Objects ```typescript const objectSchema = { type: "object", properties: { foo: { type: "string" }, bar: { type: "number" }, }, required: ["foo"], } as const; type Object = FromSchema; // => { [x: string]: unknown; foo: string; bar?: number; } ``` Defaulted properties (even optional ones) will be set as required in the resulting type. You can turn off this behavior by setting the `keepDefaultedPropertiesOptional` option to `true`: ```typescript const defaultedProp = { type: "object", properties: { foo: { type: "string", default: "bar" }, }, additionalProperties: false, } as const; type Object = FromSchema; // => { foo: string; } type Object = FromSchema< typeof defaultedProp, { keepDefaultedPropertiesOptional: true } >; // => { foo?: string; } ``` `FromSchema` partially supports the `additionalProperties`, `patternProperties` and `unevaluatedProperties` keywords: - `additionalProperties` and `unevaluatedProperties` can be used to deny additional properties. ```typescript const closedObjectSchema = { ...objectSchema, additionalProperties: false, } as const; type Object = FromSchema; // => { foo: string; bar?: number; } ``` ```typescript const closedObjectSchema = { type: "object", allOf: [ { properties: { foo: { type: "string" }, }, required: ["foo"], }, { properties: { bar: { type: "number" }, }, }, ], unevaluatedProperties: false, } as const; type Object = FromSchema; // => { foo: string; bar?: number; } ``` - Used on their own, `additionalProperties` and/or `patternProperties` can be used to type unnamed properties. ```typescript const openObjectSchema = { type: "object", additionalProperties: { type: "boolean", }, patternProperties: { "^S": { type: "string" }, "^I": { type: "integer" }, }, } as const; type Object = FromSchema; // => { [x: string]: string | number | boolean } ``` However: - When used in combination with the `properties` keyword, extra properties will always be typed as `unknown` to avoid conflicts. ```typescript const mixedObjectSchema = { type: "object", properties: { foo: { enum: ["bar", "baz"] }, }, additionalProperties: { type: "string" }, } as const; type Object = FromSchema; // => { [x: string]: unknown; foo?: "bar" | "baz"; } ``` - Due to its context-dependent nature, `unevaluatedProperties` does not type extra-properties when used on its own. Use `additionalProperties` instead. ```typescript const openObjectSchema = { type: "object", unevaluatedProperties: { type: "boolean", }, } as const; type Object = FromSchema; // => { [x: string]: unknown } ``` ## Combining schemas ### AnyOf ```typescript const anyOfSchema = { anyOf: [ { type: "string" }, { type: "array", items: { type: "string" }, }, ], } as const; type AnyOf = FromSchema; // => string | string[] ``` `FromSchema` will correctly infer factored schemas: ```typescript const factoredSchema = { type: "object", properties: { bool: { type: "boolean" }, }, required: ["bool"], anyOf: [ { properties: { str: { type: "string" }, }, required: ["str"], }, { properties: { num: { type: "number" }, }, }, ], } as const; type Factored = FromSchema; // => { // [x:string]: unknown; // bool: boolean; // str: string; // } | { // [x:string]: unknown; // bool: boolean; // num?: number; // } ``` ### OneOf `FromSchema` will parse the `oneOf` keyword in the same way as `anyOf`: ```typescript const catSchema = { type: "object", oneOf: [ { properties: { name: { type: "string" }, }, required: ["name"], }, { properties: { color: { enum: ["black", "brown", "white"] }, }, }, ], } as const; type Cat = FromSchema; // => { // [x: string]: unknown; // name: string; // } | { // [x: string]: unknown; // color?: "black" | "brown" | "white"; // } // => Error will NOT be raised 😱 const invalidCat: Cat = { name: "Garfield" }; ``` ### AllOf ```typescript const addressSchema = { type: "object", allOf: [ { properties: { address: { type: "string" }, city: { type: "string" }, state: { type: "string" }, }, required: ["address", "city", "state"], }, { properties: { type: { enum: ["residential", "business"] }, }, }, ], } as const; type Address = FromSchema; // => { // [x: string]: unknown; // address: string; // city: string; // state: string; // type?: "residential" | "business"; // } ``` ### Not Exclusions require heavy computations, that can sometimes be aborted by Typescript and end up in `any` inferred types. For this reason, they are not activated by default: You can opt-in with the `parseNotKeyword` option. ```typescript const tupleSchema = { type: "array", items: [{ const: 1 }, { const: 2 }], additionalItems: false, not: { const: [1], }, } as const; type Tuple = FromSchema; // => [] | [1, 2] ``` ```typescript const primitiveTypeSchema = { not: { type: ["array", "object"], }, } as const; type PrimitiveType = FromSchema< typeof primitiveTypeSchema, { parseNotKeyword: true } >; // => null | boolean | number | string ``` In objects and tuples, the exclusion will propagate to properties/items if it can collapse on a single one. ```typescript // 👍 Can be propagated on "animal" property const petSchema = { type: "object", properties: { animal: { enum: ["cat", "dog", "boat"] }, }, not: { properties: { animal: { const: "boat" } }, }, required: ["animal"], additionalProperties: false, } as const; type Pet = FromSchema; // => { animal: "cat" | "dog" } ``` ```typescript // ❌ Cannot be propagated const petSchema = { type: "object", properties: { animal: { enum: ["cat", "dog"] }, color: { enum: ["black", "brown", "white"] }, }, not: { const: { animal: "cat", color: "white" }, }, required: ["animal", "color"], additionalProperties: false, } as const; type Pet = FromSchema; // => { animal: "cat" | "dog", color: "black" | "brown" | "white" } ``` As some actionable keywords are not yet parsed, exclusions that resolve to `never` are granted the benefit of the doubt and omitted. For the moment, `FromSchema` assumes that you are not crafting unvalidatable exclusions. ```typescript const oddNumberSchema = { type: "number", not: { multipleOf: 2 }, } as const; type OddNumber = FromSchema; // => should and will resolve to "number" const incorrectSchema = { type: "number", not: { bogus: "option" }, } as const; type Incorrect = FromSchema; // => should resolve to "never" but will still resolve to "number" ``` Also, keep in mind that TypeScript misses [refinment types](https://en.wikipedia.org/wiki/Refinement_type): ```typescript const goodLanguageSchema = { type: "string", not: { enum: ["Bummer", "Silly", "Lazy sod !"], }, } as const; type GoodLanguage = FromSchema< typeof goodLanguageSchema, { parseNotKeyword: true } >; // => string ``` ### If/Then/Else For the same reason as the `Not` keyword, conditions parsing is not activated by default: You can opt-in with the `parseIfThenElseKeywords` option. ```typescript const petSchema = { type: "object", properties: { animal: { enum: ["cat", "dog"] }, dogBreed: { enum: Object.values(DogBreed) }, catBreed: { enum: Object.values(CatBreed) }, }, required: ["animal"], if: { properties: { animal: { const: "dog" }, }, }, then: { required: ["dogBreed"], not: { required: ["catBreed"] }, }, else: { required: ["catBreed"], not: { required: ["dogBreed"] }, }, additionalProperties: false, } as const; type Pet = FromSchema; // => { // animal: "dog"; // dogBreed: DogBreed; // catBreed?: CatBreed | undefined // } | { // animal: "cat"; // catBreed: CatBreed; // dogBreed?: DogBreed | undefined // } ``` > ☝️ `FromSchema` computes the resulting type as `(If ∩ Then) ∪ (¬If ∩ Else)`. While correct in theory, remember that the `not` keyword is not perfectly assimilated, which may become an issue in some complex schemas. ### Definitions ```typescript const userSchema = { type: "object", properties: { name: { $ref: "#/definitions/name" }, age: { $ref: "#/definitions/age" }, }, required: ["name", "age"], additionalProperties: false, definitions: { name: { type: "string" }, age: { type: "integer" }, }, } as const; type User = FromSchema; // => { // name: string; // age: number; // } ``` > ☝️ Wether in definitions or references, `FromSchema` will not work on recursive schemas for now. ### References Unlike run-time validator classes like [AJV](https://github.com/ajv-validator/ajv), TS types cannot withhold internal states. Thus, they cannot keep any identified schemas in memory. But you can hydrate them via the `references` option: ```typescript const userSchema = { $id: "http://example.com/schemas/user.json", type: "object", properties: { name: { type: "string" }, age: { type: "integer" }, }, required: ["name", "age"], additionalProperties: false, } as const; const usersSchema = { type: "array", items: { $ref: "http://example.com/schemas/user.json", }, } as const; type Users = FromSchema< typeof usersSchema, { references: [typeof userSchema] } >; // => { // name: string; // age: string; // }[] const anotherUsersSchema = { $id: "http://example.com/schemas/users.json", type: "array", items: { $ref: "user.json" }, } as const; // => Will work as well 🙌 ``` ## Deserialization You can specify deserialization patterns with the `deserialize` option: ```typescript const userSchema = { type: "object", properties: { name: { type: "string" }, email: { type: "string", format: "email", }, birthDate: { type: "string", format: "date-time", }, }, required: ["name", "email", "birthDate"], additionalProperties: false, } as const; type Email = string & { brand: "email" }; type User = FromSchema< typeof userSchema, { deserialize: [ { pattern: { type: "string"; format: "email"; }; output: Email; }, { pattern: { type: "string"; format: "date-time"; }; output: Date; }, ]; } >; // => { // name: string; // email: Email; // birthDate: Date; // } ``` ## Extensions If you need to extend the JSON Schema spec with custom properties, use the `ExtendedJSONSchema` and `FromExtendedSchema` types to benefit from `json-schema-to-ts`: ```typescript import type { ExtendedJSONSchema, FromExtendedSchema } from "json-schema-to-ts"; type CustomProps = { numberType: "int" | "float" | "bigInt"; }; const bigIntSchema = { type: "number", numberType: "bigInt", // 👇 Ensures mySchema is correct (includes extension) } as const satisfies ExtendedJSONSchema; type BigInt = FromExtendedSchema< CustomProps, typeof bigIntSchema, { // 👇 Works very well with the deserialize option! deserialize: [ { pattern: { type: "number"; numberType: "bigInt"; }; output: bigint; }, ]; } >; ``` ## Typeguards You can use `FromSchema` to implement your own typeguard: ```typescript import { FromSchema, Validator } from "json-schema-to-ts"; // It's important to: // - Explicitely type your validator as Validator // - Use FromSchema as the default value of a 2nd generic first const validate: Validator = >( schema: S, data: unknown ): data is T => { const isDataValid: boolean = ... // Implement validation here return isDataValid; }; const petSchema = { ... } as const let data: unknown; if (validate(petSchema, data)) { const { name, ... } = data; // data is typed as Pet 🙌 } ``` If needed, you can provide `FromSchema` options and additional validation options to the `Validator` type: ```typescript type FromSchemaOptions = { parseNotKeyword: true }; type ValidationOptions = [{ fastValidate: boolean }] const validate: Validator = < S extends JSONSchema, T = FromSchema >( schema: S, data: unknown, ...validationOptions: ValidationOptions ): data is T => { ... }; ``` `json-schema-to-ts` also exposes two helpers to write type guards. They don't impact the code that you wrote (they simply `return` it), but turn it into type guards. You can use them to wrap either [`validators`](#validator) or [`compilers`](#compiler). ### Validators A validator is a function that receives a schema plus some data, and returns `true` if the data is valid compared to the schema, `false` otherwise. You can use the `wrapValidatorAsTypeGuard` helper to turn validators into type guards. Here is an implementation with [ajv](https://ajv.js.org/): ```typescript import Ajv from "ajv"; import { $Validator, wrapValidatorAsTypeGuard } from "json-schema-to-ts"; const ajv = new Ajv(); // The initial validator definition is up to you // ($Validator is prefixed with $ to differ from resulting type guard) const $validate: $Validator = (schema, data) => ajv.validate(schema, data); const validate = wrapValidatorAsTypeGuard($validate); const petSchema = { ... } as const; let data: unknown; if (validate(petSchema, data)) { const { name, ... } = data; // data is typed as Pet 🙌 } ``` If needed, you can provide `FromSchema` options and additional validation options as generic types: ```typescript type FromSchemaOptions = { parseNotKeyword: true }; type ValidationOptions = [{ fastValidate: boolean }] const $validate: $Validator = ( schema, data, ...validationOptions // typed as ValidationOptions ) => { ... }; // validate will inherit from ValidationOptions const validate = wrapValidatorAsTypeGuard($validate); // with special FromSchemaOptions // (ValidationOptions needs to be re-provided) const validate = wrapValidatorAsTypeGuard< FromSchemaOptions, ValidationOptions >($validate); ``` ### Compilers A compiler is a function that takes a schema as an input and returns a data validator for this schema as an output. You can use the `wrapCompilerAsTypeGuard` helper to turn compilers into type guard builders. Here is an implementation with [ajv](https://ajv.js.org/): ```typescript import Ajv from "ajv"; import { $Compiler, wrapCompilerAsTypeGuard } from "json-schema-to-ts"; // The initial compiler definition is up to you // ($Compiler is prefixed with $ to differ from resulting type guard) const $compile: $Compiler = (schema) => ajv.compile(schema); const compile = wrapCompilerAsTypeGuard($compile); const petSchema = { ... } as const; const isPet = compile(petSchema); let data: unknown; if (isPet(data)) { const { name, ... } = data; // data is typed as Pet 🙌 } ``` If needed, you can provide `FromSchema` options, additional compiling and validation options as generic types: ```typescript type FromSchemaOptions = { parseNotKeyword: true }; type CompilingOptions = [{ fastCompile: boolean }]; type ValidationOptions = [{ fastValidate: boolean }]; const $compile: $Compiler = ( schema, ...compilingOptions // typed as CompilingOptions ) => { ... return ( data, ...validationOptions // typed as ValidationOptions ) => { ... }; }; // compile will inherit from all options const compile = wrapCompilerAsTypeGuard($compile); // with special FromSchemaOptions // (options need to be re-provided) const compile = wrapCompilerAsTypeGuard< FromSchemaOptions, CompilingOptions, ValidationOptions >($compile); ``` ## Frequently Asked Questions - [Does `json-schema-to-ts` work on _.json_ file schemas?](./documentation/FAQs/does-json-schema-to-ts-work-on-json-file-schemas.md) - [Will `json-schema-to-ts` impact the performances of my IDE/compiler?](./documentation/FAQs/will-json-schema-to-ts-impact-the-performances-of-my-ide-compiler.md) - [How can I apply `FromSchema` on generics?](./documentation/FAQs/applying-from-schema-on-generics.md) - [I get a `type instantiation is excessively deep and potentially infinite` error, what should I do?](./documentation/FAQs/i-get-a-type-instantiation-is-excessively-deep-and-potentially-infinite-error-what-should-i-do.md)

c	File Name	Size	T	Date
zip	lib/	4096	---	2026-06-01 06:57:49
-	LICENSE	1071	%	2026-06-01 06:57:47
-	README.md	26520	md	2026-06-01 06:57:47
-	package.json	3168	json	2026-06-01 06:57:47

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