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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:
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:
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>;
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>;
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>;
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: "object"
...
} as const satisfies JSONSchema
type Dog = FromSchema<typeof dogSchema>
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 = { ... }
Why use 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:
- ✅ Schema validation
FromSchema
raises TS errors on invalid schemas, based on DefinitelyTyped'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, 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:
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>;
*If json-schema-to-ts
misses one of your use case, feel free to open an issue 🤗
Table of content
Installation
npm install --save-dev json-schema-to-ts
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
.
Use cases
Const
const fooSchema = {
const: "foo",
} as const;
type Foo = FromSchema<typeof fooSchema>;
Enums
const enumSchema = {
enum: [true, 42, { foo: "bar" }],
} as const;
type Enum = FromSchema<typeof enumSchema>;
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>;
Primitive types
const primitiveTypeSchema = {
type: "null",
} as const;
type PrimitiveType = FromSchema<typeof primitiveTypeSchema>;
const primitiveTypesSchema = {
type: ["null", "string"],
} as const;
type PrimitiveTypes = FromSchema<typeof primitiveTypesSchema>;
For more complex types, refinment keywords like required
or additionalItems
will apply 🙌
Nullable
const nullableSchema = {
type: "string",
nullable: true,
} as const;
type Nullable = FromSchema<typeof nullableSchema>;
Arrays
const arraySchema = {
type: "array",
items: { type: "string" },
} as const;
type Array = FromSchema<typeof arraySchema>;
Tuples
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
FromSchema
supports the additionalItems
keyword:
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
additionalItems: false,
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
const tupleSchema = {
type: "array",
items: [{ type: "boolean" }, { type: "string" }],
additionalItems: { type: "number" },
} as const;
type Tuple = FromSchema<typeof tupleSchema>;
...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>;
Additional items will only work if Typescript's strictNullChecks
option is activated
Objects
const objectSchema = {
type: "object",
properties: {
foo: { type: "string" },
bar: { type: "number" },
},
required: ["foo"],
} as const;
type Object = FromSchema<typeof objectSchema>;
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>;
type Object = FromSchema<
typeof defaultedProp,
{ keepDefaultedPropertiesOptional: true }
>;
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>;
const closedObjectSchema = {
type: "object",
allOf: [
{
properties: {
foo: { type: "string" },
},
required: ["foo"],
},
{
properties: {
bar: { type: "number" },
},
},
],
unevaluatedProperties: false,
} as const;
type Object = FromSchema<typeof closedObjectSchema>;
- Used on their own,
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>;
However:
- When used in combination with the
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>;
- Due to its context-dependent nature,
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>;
Combining schemas
AnyOf
const anyOfSchema = {
anyOf: [
{ type: "string" },
{
type: "array",
items: { type: "string" },
},
],
} as const;
type AnyOf = FromSchema<typeof anyOfSchema>;
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>;
OneOf
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>;
const invalidCat: Cat = { name: "Garfield" };
AllOf
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>;
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.
const tupleSchema = {
type: "array",
items: [{ const: 1 }, { const: 2 }],
additionalItems: false,
not: {
const: [1],
},
} as const;
type Tuple = FromSchema<typeof tupleSchema, { parseNotKeyword: true }>;
const primitiveTypeSchema = {
not: {
type: ["array", "object"],
},
} as const;
type PrimitiveType = FromSchema<
typeof primitiveTypeSchema,
{ parseNotKeyword: true }
>;
In objects and tuples, the exclusion will propagate to properties/items if it can collapse on a single one.
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 }>;
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 }>;
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 }>;
const incorrectSchema = {
type: "number",
not: { bogus: "option" },
} as const;
type Incorrect = FromSchema<typeof incorrectSchema, { parseNotKeyword: true }>;
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 }
>;
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.
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 }>;
☝️ 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
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>;
☝️ Wether in definitions or references, FromSchema
will not work on recursive schemas for now.
References
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] }
>;
const anotherUsersSchema = {
$id: "http://example.com/schemas/users.json",
type: "array",
items: { $ref: "user.json" },
} as const;
Deserialization
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;
},
];
}
>;
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
:
import type { ExtendedJSONSchema, FromExtendedSchema } from "json-schema-to-ts";
type CustomProps = {
numberType: "int" | "float" | "bigInt";
};
const bigIntSchema = {
type: "number",
numberType: "bigInt",
} as const satisfies ExtendedJSONSchema<CustomProps>;
type BigInt = FromExtendedSchema<
CustomProps,
typeof bigIntSchema,
{
deserialize: [
{
pattern: {
type: "number";
numberType: "bigInt";
};
output: bigint;
},
];
}
>;
Typeguards
You can use FromSchema
to implement your own typeguard:
import { FromSchema, Validator } from "json-schema-to-ts";
const validate: Validator = <S extends JSONSchema, T = FromSchema<S>>(
schema: S,
data: unknown
): data is T => {
const isDataValid: boolean = ...
return isDataValid;
};
const petSchema = { ... } as const
let data: unknown;
if (validate(petSchema, data)) {
const { name, ... } = data;
}
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
.
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:
import Ajv from "ajv";
import { $Validator, wrapValidatorAsTypeGuard } from "json-schema-to-ts";
const ajv = new Ajv();
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;
}
If needed, you can provide FromSchema
options and additional validation options as generic types:
type FromSchemaOptions = { parseNotKeyword: true };
type ValidationOptions = [{ fastValidate: boolean }]
const $validate: $Validator<ValidationOptions> = (
schema,
data,
...validationOptions
) => { ... };
const validate = wrapValidatorAsTypeGuard($validate);
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:
import Ajv from "ajv";
import { $Compiler, wrapCompilerAsTypeGuard } from "json-schema-to-ts";
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;
}
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: $Compiler<CompilingOptions, ValidationOptions> = (
schema,
...compilingOptions
) => {
...
return (
data,
...validationOptions
) => { ... };
};
const compile = wrapCompilerAsTypeGuard($compile);
const compile = wrapCompilerAsTypeGuard<
FromSchemaOptions,
CompilingOptions,
ValidationOptions
>($compile);
Frequently Asked Questions