desy

desy - Dead Extraordinary Simple Yup

• Install
• Basic usage
• Key ideas
• Key features
• Examples
• API
• JSON Schema
• Benchmark

Install

npm install desy

Basic usage

Creating a simple string schema

import {d} from 'desy';

// creating a schema for strings
const mySchema = d.string();

// validating
mySchema.validate('tuna'); // => ""
mySchema.validate(12); // => "Value must be string"

Key ideas

• be simple
• be as fast as point 1 allows

Key features

• Stop validating on the first error. Desy stops validating on the first error and returns it.
• A string is an indicator. Desy returns an empty string in a valid case. In case of an error, Desy returns a string with a description of the error.
• No throwing errors. Desy only returns an empty or non-empty string.

Examples

Deep schema

const schema = d.array(
  d.object({
    type: d.string().oneOf(['person']),
    hair: d.string().oneOf(['blue', 'brown']),
    active: d.boolean(),
    name: d.string(),
    age: d.number().int(),
    hobbies: d.array(d.string()),
    address: d.object({
      street: d.string(),
      zip: d.string(),
      country: d.string(),
    }),
  }),
);
schema.validate(people);

Get schema's types

import {d, InferDesy} from 'desy';

const userSchema = d.object({
  username: d.string(),
});

const error = userSchema.validate({username: 'Ludwig'}); // error is ""

// extract the inferred type
type User = InferDesy<typeof user>; // { username: string }

API

• Common
• mixed()
• string()
• number()
• number()
• boolean()
• date()
• null()
• object()
• array()

Common

• validate(value: any)

const schema = d.mixed();

schema.validate('a'); // valid
schema.validate(''); // valid
schema.validate(null); // valid

• .test(func: (value: sting) => string)

const schema = d.mixed().test((value) => {
  if (value === 'world') {
    return ''; // valid case
  }

  return 'MUST BE WORLD'; // error message
});

schema.validate('hello'); // error
schema.validate('world'); // valid

• .description(text: string)

Describes the schema for a reader. Ignored by validate, used by createJsonSchema.

const schema = d.string().description('Full name as in the passport');

schema.validate('John'); // valid

mixed

• mixed()

const schema = d.mixed();

schema.validate('a'); // valid
schema.validate(''); // valid
schema.validate(null); // valid

string

• .string()

const schema = d.string();

schema.validate('a'); // valid
schema.validate(''); // error
schema.validate(null); // error

• .length(chars)

const schema = d.string().length(1);

schema.validate('aa'); // error
schema.validate('a'); // valid

• .optional()

const schema = d.string().optional();

schema.validate(''); // valid
schema.validate('a'); // valid

• .oneOf(variants: string[])

const schema = d.string().oneOf(['hello', 'world']);

schema.validate('hello'); // valid
schema.validate('world'); // valid
schema.validate('foo'); // error

• .regexp(regexp: Regexp)

const schema = d.string().regexp(/hello/i);

schema.validate('123hello'); // valid
schema.validate('hell'); // error

• .min(min_chars: number)

const schema = d.string().min(1);

schema.validate(''); // error
schema.validate('a'); // valid

• .max(max_chars: number)

const schema = d.string().max(1);

schema.validate('aa'); // error
schema.validate('a'); // valid

number

• .number()

const schema = d.number();

schema.validate(42); // valid
schema.validate('42'); // error

• .int()

const schema = d.number().int();

schema.validate(42); // valid
schema.validate(42.2); // error

• .float()

const schema = d.number().float();

schema.validate(42); // error
schema.validate(42.2); // valid

• .min(num: number)

const schema = d.number().min(1);

schema.validate(0); // error
schema.validate(1); // valid

• .max(num: number)

const schema = d.number().max(1);

schema.validate(1); // valid
schema.validate(2); // error

boolean

• .boolean()

const schema = d.boolean();

schema.validate(true); // valid
schema.validate(false); // valid
schema.validate(1); // error

• .true()

const schema = d.boolean().true();

schema.validate(true); // valid
schema.validate(false); // error

• .false()

const schema = d.boolean().false();

schema.validate(true); // error
schema.validate(false); // valid

date

• .date()

const schema = d.date();

schema.validate(0); // valid
schema.validate('2024-03-15T23:21:48.605Z'); // valid
schema.validate(new Date()); // valid
schema.validate(undefined); // error

• .min(date: DateValue)

const now = Date.now();
const schema = d.date().min(now);

schema.validate(now); // valid
schema.validate(now - 1); // error

• .max(date: DateValue)

const now = Date.now();
const schema = d.date().max(now);

schema.validate(now); // valid
schema.validate(now + 1); // error

null

• .null()

const schema = d.null();

schema.validate(null); // valid
schema.validate(undefined); // error

object

• .object(objschema: Record<string, Schema>)

const schema = d.object({
  name: d.sting(),
});

schema.validate({name: 'alex'}); // valid
schema.validate({name: 'alex', age: 42}); // error
schema.validate({name: 42}); // error

• .notStrict()

const schema = d
  .object({
    name: d.sting(),
  })
  .notStrict();

schema.validate({name: 'alex'}); // valid
schema.validate({name: 'alex', age: 42}); // valid
schema.validate({name: 42}); // error

• .optionalFields(fileds: string[])

const schema = d
  .object({
    name: d.sting(),
  })
  .optionalFields(['name']);

schema.validate({name: 'alex'}); // valid
schema.validate({}); // valid
schema.validate({name: 42}); // error

array

• .array(schemas: Schema[])

const schema = d.arrar(d.sting());

schema.validate(['hello', 'world']); // valid
schema.validate(['hello', 42]); // error

• .length(length: number)

const schema = d.arrar(d.sting()).length(2);

schema.validate(['hello', 'world']); // valid
schema.validate(['world']); // error

• .min(min_length: number)

const schema = d.arrar(d.sting()).min(2);

schema.validate(['hello', 'world']); // valid
schema.validate(['world']); // error

• .max(max_length: number)

const schema = d.arrar(d.sting()).max(2);

schema.validate(['hello', 'world']); // valid
schema.validate(['hello', 'world', 'foo']); // error

JSON Schema

createJsonSchema(schema) turns a desy schema into a JSON Schema (draft 2020-12). Useful when the data you are about to validate comes from an LLM: the same schema constrains the generation and checks the answer.

import {d, createJsonSchema} from 'desy';

const schema = d
  .object({
    sentiment: d.string().oneOf(['positive', 'negative']),
    score: d.number().int().min(1).max(5).description('From 1 to 5'),
    reply: d.string().undefinable(),
  })
  .description('Review breakdown');

createJsonSchema(schema);

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "additionalProperties": false,
  "properties": {
    "sentiment": {
      "type": "string",
      "minLength": 1,
      "enum": ["positive", "negative"]
    },
    "score": {
      "type": "integer",
      "minimum": 1,
      "maximum": 5,
      "description": "From 1 to 5"
    },
    "reply": {"type": "string", "minLength": 1}
  },
  "required": ["sentiment", "score"],
  "description": "Review breakdown"
}

Worth knowing:

• d.string() becomes minLength: 1 — an empty string is not valid in desy.
• additionalProperties is always false, whether or not strictObject() was called. Strict structured outputs require it, and a schema stricter than the validator never rejects what desy accepts.
• A field is in required unless its schema is undefinable() — JSON has no undefined, absence is expressed by required alone.
• nullable() becomes "type": ["string", "null"].
• Repeated constraints narrow instead of overwriting: .min(2).min(5) gives minLength: 5, two oneOf intersect. A second regexp on the same string cannot be narrowed and throws.

Five rules do not convert and throw instead of being dropped: .test(), number().float(), date().min(), date().max() and mixed().notVoid(). Describe such a limit with .description() so the model still learns about it.

benchmark

	Simple string result	Complex object result
desy	x	x
zod	10x	8x
yup	43x	31x

smaller is better