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json-schema-to-ts
Advanced tools
The json-schema-to-ts npm package is a utility that converts JSON Schema definitions into TypeScript types. This allows developers to ensure type safety and consistency between their JSON data and TypeScript code.
Convert JSON Schema to TypeScript Types
This feature allows you to convert a JSON Schema into a TypeScript type. The `FromSchema` utility type takes a JSON Schema object and infers the corresponding TypeScript type.
import { FromSchema } from 'json-schema-to-ts';
const userSchema = {
type: 'object',
properties: {
id: { type: 'string' },
name: { type: 'string' },
age: { type: 'number' }
},
required: ['id', 'name']
} as const;
type User = FromSchema<typeof userSchema>;
const user: User = {
id: '123',
name: 'John Doe',
age: 30
};
Type Validation
This feature ensures that the TypeScript types derived from JSON Schema are validated at compile time, reducing runtime errors and improving code reliability.
import { FromSchema } from 'json-schema-to-ts';
const productSchema = {
type: 'object',
properties: {
productId: { type: 'string' },
price: { type: 'number' }
},
required: ['productId', 'price']
} as const;
type Product = FromSchema<typeof productSchema>;
const product: Product = {
productId: 'abc123',
price: 19.99
};
The typescript-json-schema package generates JSON Schema from your TypeScript types. It works in the opposite direction compared to json-schema-to-ts, which converts JSON Schema to TypeScript types. This package is useful if you start with TypeScript types and need to generate JSON Schema for validation or documentation purposes.
Ajv is a JSON Schema validator that can also generate TypeScript types from JSON Schema using additional plugins. While its primary focus is on validation, it offers similar functionality to json-schema-to-ts when combined with the appropriate plugins.
Zod is a TypeScript-first schema declaration and validation library. It allows you to define schemas in TypeScript and infer types from them. While it doesn't convert JSON Schema to TypeScript types directly, it provides a similar type-safe experience for defining and validating data structures.
💖 Huge thanks to the sponsors who help me maintain this repo:
Theodo | Your Brand Here |
A lot of projects use JSON schemas for runtime data validation along with TypeScript for static type checking.
Their code may look like this:
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 💪
The FromSchema
method lets you infer TS types directly from JSON schemas:
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<typeof dogSchema>;
// => Will infer the same type as above
Schemas can even be nested, as long as you don't forget the as const
statement:
const catSchema = { ... } as const;
const petSchema = {
anyOf: [dogSchema, catSchema],
} as const;
type Pet = FromSchema<typeof petSchema>;
// => 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.
import { asConst } from "json-schema-to-ts";
const dogSchema = asConst({
type: "object",
...
});
type Dog = FromSchema<typeof dogSchema>;
// => Will work as well 🙌
Since TS 4.9, you can also use the satisfies
operator to benefit from type-checking and autocompletion:
import type { JSONSchema } from "json-schema-to-ts";
const dogSchema = {
// Type-checked and autocompleted 🙌
type: "object"
...
} as const satisfies JSONSchema
type Dog = FromSchema<typeof dogSchema>
// => 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<typeof dogSchema>} */
const dog = { ... }
json-schema-to-ts
?If you're looking for runtime validation with added types, libraries like yup, zod or 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:
FromSchema
raises TS errors on invalid schemas, based on DefinitelyTyped's definitionsjson-schema-to-ts
only operates in type space. And after all, what's lighter than a dev-dependency?string
that you used instead of an enum
? Or this additionalProperties
you confused with additionalItems
? Or forgot entirely? Well, json-schema-to-ts
does!FromSchema
is extensively tested against AJV, and covers all the use cases that can be handled by TS for now*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
!
type Address = FromSchema<typeof addressSchema>;
// => never 🙌
*If
json-schema-to-ts
misses one of your use case, feel free to open an issue 🤗
# 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+. Usingstrict
mode is required, as well as (apparently) turning offnoStrictGenericChecks
.
const fooSchema = {
const: "foo",
} as const;
type Foo = FromSchema<typeof fooSchema>;
// => "foo"
const enumSchema = {
enum: [true, 42, { foo: "bar" }],
} as const;
type Enum = FromSchema<typeof enumSchema>;
// => true | 42 | { foo: "bar"}
You can also go full circle with typescript enums
.
enum Food {
Pizza = "pizza",
Taco = "taco",
Fries = "fries",
}
const enumSchema = {
enum: Object.values(Food),
} as const;
type Enum = FromSchema<typeof enumSchema>;
// => Food
const primitiveTypeSchema = {
type: "null", // "boolean", "string", "integer", "number"
} as const;
type PrimitiveType = FromSchema<typeof primitiveTypeSchema>;
// => null, boolean, string or number
const primitiveTypesSchema = {
type: ["null", "string"],
} as const;
type PrimitiveTypes = FromSchema<typeof primitiveTypesSchema>;
// => null | string
For more complex types, refinment keywords like
required
oradditionalItems
will apply 🙌
const nullableSchema = {
type: "string",
nullable: true,
} as const;
type Nullable = FromSchema<typeof nullableSchema>;
// => string | null
const arraySchema = {
type: "array",
items: { type: "string" },
} as const;
type Array = FromSchema<typeof arraySchema>;
// => string[]
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string] | [boolean, string, ...unknown[]]
FromSchema
supports the additionalItems
keyword:
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
additionalItems: false,
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string]
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
additionalItems: { type: "number" },
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string] | [boolean, string, ...number[]]
...as well as the minItems
and maxItems
keywords:
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
minItems: 1,
maxItems: 2,
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
// => [boolean] | [boolean, string]
Additional items will only work if Typescript's
strictNullChecks
option is activated
const objectSchema = {
type: "object",
properties: {
foo: { type: "string" },
bar: { type: "number" },
},
required: ["foo"],
} as const;
type Object = FromSchema<typeof objectSchema>;
// => { [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
:
const defaultedProp = {
type: "object",
properties: {
foo: { type: "string", default: "bar" },
},
additionalProperties: false,
} as const;
type Object = FromSchema<typeof defaultedProp>;
// => { 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.const closedObjectSchema = {
...objectSchema,
additionalProperties: false,
} as const;
type Object = FromSchema<typeof closedObjectSchema>;
// => { foo: string; bar?: number; }
const closedObjectSchema = {
type: "object",
allOf: [
{
properties: {
foo: { type: "string" },
},
required: ["foo"],
},
{
properties: {
bar: { type: "number" },
},
},
],
unevaluatedProperties: false,
} as const;
type Object = FromSchema<typeof closedObjectSchema>;
// => { foo: string; bar?: number; }
additionalProperties
and/or patternProperties
can be used to type unnamed properties.const openObjectSchema = {
type: "object",
additionalProperties: {
type: "boolean",
},
patternProperties: {
"^S": { type: "string" },
"^I": { type: "integer" },
},
} as const;
type Object = FromSchema<typeof openObjectSchema>;
// => { [x: string]: string | number | boolean }
However:
properties
keyword, extra properties will always be typed as unknown
to avoid conflicts.const mixedObjectSchema = {
type: "object",
properties: {
foo: { enum: ["bar", "baz"] },
},
additionalProperties: { type: "string" },
} as const;
type Object = FromSchema<typeof mixedObjectSchema>;
// => { [x: string]: unknown; foo?: "bar" | "baz"; }
unevaluatedProperties
does not type extra-properties when used on its own. Use additionalProperties
instead.const openObjectSchema = {
type: "object",
unevaluatedProperties: {
type: "boolean",
},
} as const;
type Object = FromSchema<typeof openObjectSchema>;
// => { [x: string]: unknown }
const anyOfSchema = {
anyOf: [
{ type: "string" },
{
type: "array",
items: { type: "string" },
},
],
} as const;
type AnyOf = FromSchema<typeof anyOfSchema>;
// => string | string[]
FromSchema
will correctly infer factored schemas:
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<typeof factoredSchema>;
// => {
// [x:string]: unknown;
// bool: boolean;
// str: string;
// } | {
// [x:string]: unknown;
// bool: boolean;
// num?: number;
// }
FromSchema
will parse the oneOf
keyword in the same way as anyOf
:
const catSchema = {
type: "object",
oneOf: [
{
properties: {
name: { type: "string" },
},
required: ["name"],
},
{
properties: {
color: { enum: ["black", "brown", "white"] },
},
},
],
} as const;
type Cat = FromSchema<typeof catSchema>;
// => {
// [x: string]: unknown;
// name: string;
// } | {
// [x: string]: unknown;
// color?: "black" | "brown" | "white";
// }
// => Error will NOT be raised 😱
const invalidCat: Cat = { name: "Garfield" };
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<typeof addressSchema>;
// => {
// [x: string]: unknown;
// address: string;
// city: string;
// state: string;
// type?: "residential" | "business";
// }
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.
const tupleSchema = {
type: "array",
items: [{ const: 1 }, { const: 2 }],
additionalItems: false,
not: {
const: [1],
},
} as const;
type Tuple = FromSchema<typeof tupleSchema, { parseNotKeyword: true }>;
// => [] | [1, 2]
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.
// 👍 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<typeof petSchema, { parseNotKeyword: true }>;
// => { animal: "cat" | "dog" }
// ❌ 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<typeof petSchema, { parseNotKeyword: true }>;
// => { 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.
const oddNumberSchema = {
type: "number",
not: { multipleOf: 2 },
} as const;
type OddNumber = FromSchema<typeof oddNumberSchema, { parseNotKeyword: true }>;
// => should and will resolve to "number"
const incorrectSchema = {
type: "number",
not: { bogus: "option" },
} as const;
type Incorrect = FromSchema<typeof incorrectSchema, { parseNotKeyword: true }>;
// => should resolve to "never" but will still resolve to "number"
Also, keep in mind that TypeScript misses refinment types:
const goodLanguageSchema = {
type: "string",
not: {
enum: ["Bummer", "Silly", "Lazy sod !"],
},
} as const;
type GoodLanguage = FromSchema<
typeof goodLanguageSchema,
{ parseNotKeyword: true }
>;
// => string
For the same reason as the Not
keyword, conditions parsing is not activated by default: You can opt-in with the parseIfThenElseKeywords
option.
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<typeof petSchema, { parseIfThenElseKeywords: true }>;
// => {
// 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 thenot
keyword is not perfectly assimilated, which may become an issue in some complex schemas.
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<typeof userSchema>;
// => {
// name: string;
// age: number;
// }
☝️ Wether in definitions or references,
FromSchema
will not work on recursive schemas for now.
Unlike run-time validator classes like 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:
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 🙌
You can specify deserialization patterns with the deserialize
option:
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;
// }
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
:
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<CustomProps>;
type BigInt = FromExtendedSchema<
CustomProps,
typeof bigIntSchema,
{
// 👇 Works very well with the deserialize option!
deserialize: [
{
pattern: {
type: "number";
numberType: "bigInt";
};
output: bigint;
},
];
}
>;
You can use FromSchema
to implement your own typeguard:
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 = <S extends JSONSchema, T = FromSchema<S>>(
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:
type FromSchemaOptions = { parseNotKeyword: true };
type ValidationOptions = [{ fastValidate: boolean }]
const validate: Validator<FromSchemaOptions, ValidationOptions> = <
S extends JSONSchema,
T = FromSchema<S, FromSchemaOptions>
>(
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
or compilers
.
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:
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:
type FromSchemaOptions = { parseNotKeyword: true };
type ValidationOptions = [{ fastValidate: boolean }]
const $validate: $Validator<ValidationOptions> = (
schema,
data,
...validationOptions // typed as ValidationOptions
) => { ... };
// validate will inherit from ValidationOptions
const validate = wrapValidatorAsTypeGuard($validate);
// with special FromSchemaOptions
// (ValidationOptions needs to be re-provided)
const validate = wrapValidatorAsTypeGuard<
FromSchemaOptions,
ValidationOptions
>($validate);
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:
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:
type FromSchemaOptions = { parseNotKeyword: true };
type CompilingOptions = [{ fastCompile: boolean }];
type ValidationOptions = [{ fastValidate: boolean }];
const $compile: $Compiler<CompilingOptions, ValidationOptions> = (
schema,
...compilingOptions // typed as CompilingOptions
) => {
...
return (
data,
...validationOptions // typed as ValidationOptions
) => { ... };
};
// compile will inherit from all options
const compile = wrapCompilerAsTypeGuard($compile);
// with special FromSchemaOptions
// (options need to be re-provided)
const compile = wrapCompilerAsTypeGuard<
FromSchemaOptions,
CompilingOptions,
ValidationOptions
>($compile);
FAQs
Infer typescript types from your JSON schemas!
The npm package json-schema-to-ts receives a total of 1,648,225 weekly downloads. As such, json-schema-to-ts popularity was classified as popular.
We found that json-schema-to-ts demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 1 open source maintainer collaborating on the project.
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