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Simple, typescript-first schema validation tool

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# Jet-Schema ✈️

> Simple, zero-dependency, typescript-first schema validation tool, with zero-overhead when it comes to custom validation.

---

## Table of contents
- [Introduction](#introduction)
- [Comparison to other schema validation libraries](#comparison-to-others)
- [Guide](#guide)
 - [Installation](#installation)
 - [Creating Schemas](#creating-schemas)
 - [Passing validator-functions](#passing-validator-functions)
 - [The IError object](#ierror-object)
 - [The jetSchema function](#the-jet-schema-function)
 - [The jetSchema function's additional options](#jet-schema-additional-options)
 - [The standalone schema function](#the-schema-function)
 - [Handling a schema's type](#handling-the-schemas-type)
 - [The "options" param](#schema-options)
 - [Schema APIs](#schema-apis)
 - [.new](#new)
 - [.test](#test)
 - [.pick](#pick)
 - [.parse](#parse)
 - [Combining Schemas](#combining-schemas)
 - [TypeScript Caveats](#typescript-caveats)
 - [Bonus Features](#bonus-features)
 - [Using jet-schema without TypeScript](#without-typescript)
- [Tips](#tips)
 - [Creating wrapper functions](#creating-wrapper-functions)
 - [Recommended Global Settings](#recommended-global-settings)
 - [jet-validators](#jet-validators)

## Introduction <a name="introduction"></a>
`jet-schema` is a simple, TypeScript-first schema validation tool which enables you to use your own validator functions against each property in an object.

> If you're open to `jet-schema` but think writing your own validator-functions could be a hassle, you install its sister library <a href="https://github.com/seanpmaxwell/jet-validators">jet-validators</a> which includes some predefined validator-functions.

### Highlights 😎
- Focus on using your own validator-functions to validate object properties.
- Enable extracting logic for nested schemas.
- Create new instances of your schemas using partials.
- Quickly learn this terse, small library that only exports 2 functions and 3 types, size **4.7kB** minified.
- Doesn't require a compilation step (so still works with `ts-node`, unlike `typia`).
- Fast! see these <a href="https://moltar.github.io/typescript-runtime-type-benchmarks/">benchmarks</a>.
- Typesafety works boths ways, you can infer a type from a schema or force a schema to have certain properties using a generic. 
- Works client-side or server-side.
- TypeScript first!
- Sample project using jet-schema <a href="https://github.com/seanpmaxwell/express5-typescript-template/blob/master/src/models/User.ts">here</a>.

### Quick Glance
```typescript
import schema, { inferType } from 'utils/schema';
import { isString, isNumber } from 'utils/validators';

const User = schema({
  id: isNumber,
  name: isString,
  address: schema({
    street: isString,
    zip: isNumber,
  })
});

User.new({ id: 5, name: 'joe' }); // => { id: 5, name: 'joe' }
User.test('asdf'); // => false
User.pick('name').test('john'); // => true
User.pick('address').pick('zip').test(234); // => true
User.parse('something'); // => Error
type TUser = inferType<User>; // Get the type
```

## Comparison to other schema validation libraries <a name="comparison-to-others"></a>

### What is a validator function?
A validator-function is a TypeScript function which does both *runtime* AND *compile-time* validation. The typical way to define one is to give it a signature which receives an `unknown` value and returns a *type-predicate*:
```typescript
function isNullishString(arg: unknown): param is string | undefined | null {
  return arg === undefined || arg === null || typeof arg === 'string';
}
```

### Why keep your own list of validator-functions?
When validating an individual object's property, there's literally an infinite list of validations that can be done which are specific to that application needs (i.e. different businesses might have different requirements for an email format). So I thought, why not just strip all that away and just make something that allows you to use existing validator-functions to check an object's properties?

Other reasons to keep your own list of validator-functions:
- Reuse your validators in multiple parts of your code across multiple projects without worrying about which library they are tied to.
- Reduce the need to repeatedly wrap your specialized logic in a library's handlers (i.e. zod's `.refine` function).
- Name your validator-functions however you want (for example: I like to use abbreviations a lot).
- Make your code substantially more terse.
- Setup your schema and never refer to the library's documentation again.`jet-schema` takes a *fire-and-forget* approach
- As mentioned in the intro, you can skip having to defined some of the more common ones by installing <a href="https://github.com/seanpmaxwell/jet-validators">jet-validators</a>.

### Code comparison with zod and jet-schema
```typescript
import { isString, isNumber, isRelationalKey, isEmail, isOptionalString } from 'your-validators.ts';

interface IUser {
  id: number;
  name: string;
  email: string;
  age: number;
  created: Date;
  address?: {
    street: string;
    zip: number;
    country?: string;
  };
}

// "zod"
const User: z.ZodType<IUser> = z.object({
 id: z.number().default(-1).min(-1),
 name: z.string().default(''),
 email: z.string().email().or(z.literal('')).default('x@x.x'),
 // OR if we had our own "isEmail" validator-function
 email: z.string().refine(val => isEmail(val)).default('x@x.x'),
 age: z.preprocess(Number, z.number()),
 created: z.preprocess((arg => arg === undefined ? new Date() : arg), z.coerce.date()),
 address: z.object({ 
 street: z.string(),
 zip: z.number(),
 country: z.string().optional(),
 }).optional(),
});

// "jet-schema"
const User = schema<IUser>({
 id: isRelationalKey,
 name: isString,
 email: { vf: isEmail, default: 'x@x.x' },
 age: { vf: isNumber, transform: Number },
 created: Date,
 address: schema({
 street: isString,
 zip: isNumber,
 country: isOptionalString,
 }, { optional: true }),
});
```

### Size (minified, not zipped)
- Jet-Schema **5kB**
- Zod **57kB**
- Yup **40kB**
- Joi **150kB**
- Valibot **35kB**
- Note: some of these could change as packages are updated

### Fast
- See these benchmarks <a href="https://moltar.github.io/typescript-runtime-type-benchmarks/">here</a>.
- Looking at the benchmarks in the link above, compare jet-schema to some other popular validators and notice that it is rougly 2-3 times as fast as the ones which don't require a compilation setup (i.e. zod, valibot, yup etc).

### Create instances with partials
Another major reason I created jet-schema was to create multiple instances of my schemas using partials and copies of existing instances when doing edits. This can be done with the `.new` function (see the <a name="new">.new</a> section for more details).

## Guide <a name="guide"></a>

### Installation <a name="installation"></a>
> npm install -s jet-schema

### Creating schemas <a name="creating-schemas"></a>

#### ▸ Passing validator-functions <a name="passing-validator-functions"></a>
Validator-functions can be passed to schemas directly or within a *configuration-object*. These objects allow us to handle settings for individual validator-functions:
```typescript
// configuration-object format:
{
 vf: <T>(arg: unknown) => arg is T; // vf => validator-function 
 default?: T; // the default value for the validator-function
 transform?: (arg: unknown) => T; // modify the value before calling the validator-function
 formatError?: (error: IError) => IError | string; // Customize what's sent to onError() when errors are raised.
}

// Example
const UserSchema = schema({
  name: isString, // Using a validator-function directly
  id: {  // Using a configuration-object
    vf: isNumber, // the validator-function in the object
    default: 0,
    transform: Number,
    formatError: err => `Property ${err.property} was not a valid number`,
  },
});
```

#### ▸ IError object <a name="ierror-object"></a>
In the previous snippet we see the `formatError` function passes an `IError` object. The format for an `IError` object is:
```typescript
{
 property?: string;
 value?: unknown;
 message?: string;
 location?: string; // function which is throwing the error
 schemaId?: string;
}
```

#### ▸ The jetSchema function <a name="the-jet-schema-function"></a>
Schemas can be created by importing the `schema` function directly from the `jet-schema` library or importing the default `jetSchema` function. The `jetSchema` function can be passed an array of configuration-objects and returns a new customized `schema` function; that way we don't have to configure validator-function settings for every new schema.

The configuration-objects are set in the `globals:` property. Note that localized settings will overwrite all global ones:
```typescript
import jetSchema from 'jet-schema';
import { isNumber, isString } from './validators';

const schema = jetSchema({
  globals?: [
    { vf: isNumber, default: 0 },
    { vf: isString, default: '' },
  ],
});

const User1 = schema({
  id: isNumber,
  name: isString,
});

const User2 = schema({
  id: { vf: isNumber, default: -1 }, // Localized default setting overwriting a global one
  name: isString,
});

User1.new(); // => { id: 0, name: '' }
User2.new(); // => { id: -1, name: '' }
```

#### ▸ The jetSchema function's additional options <a name="jet-schema-additional-options"></a>
For the `jetSchema` function, in addition to `globals:` there are two additional options we can configure:
- `cloneFn`: A custom clone-function. When using the `.new` function, all partial values will be cloned. The default clone function uses `structuredClone` (I like to use `lodash.cloneDeep`).
- `onError`: Configure what happens when errors are raised. By default, a javascript `new Error()` is thrown with the array of errors stringified in the error message.
```typescript
import jetSchema from 'jet-schema';
import { isNumber, isString } from './validators';

export default jetSchema({
  globals?: [
    { vf: isNumber, default: 0 },
    { vf: isString, default: '' },
  ],
  cloneFn?: (val: unknown) => unknown, // use a custom clone-function
  onError?: (errors: IError[]) => void, // pass a custom error-handler,
});
```

> I usually configure the `jetSchema` function once per application and place it in a script called `utils/schema.ts`. From there I import it and use it to configure all individual schemas: take a look at this <a href="https://github.com/seanpmaxwell/express5-typescript-template/tree/master">template</a> for an example.

#### ▸ The standalone schema function <a name="the-schema-function"></a>
If we did not use the `jetSchema` function above and instead used the `schema` function directly, default values would have to be reconfigured everytime. **IMPORTANT** If your validator-function does not accept `undefined` as a valid value, you must set a default value because all defaults will be validated at startup:
 ```typescript
import { schema } from 'jet-schema';
import { isNumber, isString } from './validators';

const User1 = schema({
  id: { vf: isNumber, default: 0 },
  name: { vf: isString, default: '' },
});

const User2 = schema({
  id: { vf: isNumber, default: 0 },
  name: isString, // ERROR: "isString" does not accept `undefined` as a valid value but no default value was configured for "isString"
});
```

#### ▸ Handling a schema's type <a name="handling-the-schemas-type"></a>
For handling a schema's type, you can enforce a schema from a type or infer a type from a schema.

**Option 1:** Create a schema using a type:
```typescript
import { schema } from 'jet-schema';
import { isNumber, isString, isOptionalString } from 'util/validators.ts';

interface IUser {
  id: number;
  name: string;
  email?: string;
}

const User = schema<IUser>({
 id: isNumber,
 name: isString,
 email: isOptionalString,
});
```

**Option 2:** Create a type using a schema:
```typescript
import { schema, inferType } from 'jet-schema';
import { isNumber, isString, isOptionalString } from 'util/validators.ts';

const User = schema({
  id: isNumber,
  name: isString,
  email: isOptionalString,
});

const TUser = inferType<typeof User>;
```

#### ▸ The "options" param <a name="schema-options"></a>
In addition to an object with our schema's properties, the `schema` function accepts an additional **options** parameter:
```typescript
const User = schema<IUser>({
 id: isNumber,
 name: isString,
}, /* { ...options object... } */); // <-- Pass options here
```

`options` explained:
  - `optional`: Default `false`, must be set to true if the generic is optional.
  - `nullable`: Default `false`, must be set to true if the generic is nullable.
  - `nullish`: Default `false`, convenient alternative to `{ optional: true, nullable: true; }`
  - `init`: Tells the parent what to do when the parent calls `.new`.
    - `false`: Skip creating a child-object. The child-object must be `optional`.
    - `true`: Create a new child-object (Uses the child's `.new` function).
    - `null`: Set the child object's value to `null` (`nullable` must be true for the child).
  - `id`: A unique-identifier for the schema passed to the `IError` object.
  - `safety`: Sets how to deal with additional properties. 
    - `'filter' (default)`: Properties not in the schema will be filtered out but not raise errors.
    - `'pass'`: Properties not in the schema will not be filtered out nor raise errors.
    - `'strict'`: Properties not in the schema will be filtered out and raise errors.
    - **NOTE:** `safety` only applies to the `.test` and `.parse` functions, it does not affect `.new`.

`options` example:
```typescript
type TUser = IUser | null | undefined;

const User = schema<TUser>({
 id: isNumber,
 name: isString,
}, {
 optional: true, // Must be true because TUser is `| undefined`
 nullable: true, // Must be true because TUser is `| null`
 nullish: true, // Alternative to { optional: true, nullable: true }
 init: false, // Can be "null", "false", or "true"
 id: 'User',
 safety: 'strict'
});
```

### Schema APIs <a name="schema-apis"></a>
Once you have your custom schema setup, you can call the `.new`, `.test`, `.pick`, and `.parse` functions.

> NOTE: the following examples assume you set `0` as the default for `isNumber`, `''` for `isString`, nothing for `isOptionalString`, and `safety` is left at its default `filter` option. See the <a name="creating-schemas">Creating Schemas</a> section for how to set default values and the `safety` option.

#### `.new` <a name="new"></a>
Allows you to create new instances of your type using partials. If the property is absent, `.new` will use the default supplied. If no default is supplied and the property is optional, then the value will be skipped. Runtime validation will still be done on every incoming property:
```typescript
User.new(); // => { id: 0, name: '' }
User.new({ id: 5 }); // => { id: 5, name: '' }
User.new({ id: 'asdf' }); // => Error
User.new({ name: 'john' }); // => { id: 0, name: 'john' }
User.new({ id: 1, name: 'a', email: 'b@b' }); // => { id: 1, name: 'a', email: 'b@b' }
```

#### `.test` <a name="test"></a>
Accepts any unknown value, tests that it's valid, and returns a type-predicate:
```typescript
User.test(); // => Error
User.test({ id: 5, name: 'john' }); // => param is IUser
User.test({ name: 'john' }); // => Error
User.test({ id: 1, name: 'a', email: 'b@b' }); // => param is IUser
```

#### `.pick` <a name="pick"></a>
Selects a property and returns an object with the `.test` and `.default` functions. If you use `.pick` on a child schema, you can also use the schema functions (`.new`, `.pick` etc), in addition to `.default`. Note that for a child-schema, `.default` could return a different value from `.new` if the default value is set to `null` or `undefined` (see the `init:` setting in the <a href="#schema-options">Schema Options</a> section).
```typescript
const User = schema<IUser>({
 id: isNumber,
 address: schema<IUser['address']>({
 street: isString,
 city: isString,
 }, { init: null }),
});

User.pick('id').default(); // => "0"
User.pick('id').test(0); // => "true"
User.pick('id').test('asdf'); // => "false"
User.pick('address').new(); // => { street: '', city: '' }
User.pick('address').default(); // => "null"
User.pick('address').pick('city').test('asdf'); // => "true"
```

#### `.parse` <a name="parse"></a>
Like a combination of `.new` and `.test`. It accepts an `unknown` value which is not optional, validates the properties but returns a new instance (while removing an extra ones) instead of a type-predicate. Note: only objects will pass the `.parse` function, even if a schema is nullish, `null/undefined` values will not pass.
```typescript
const User = schema<IUser>({
 id: isNumber,
 name: isString,
});

User.parse(); // => Error
User.parse({ id: 1, name: 'john' }); // => { id: 1, name: 'john' }
User.parse({ id: 1, name: 'john', foo: 'bar' }); // => { id: 1, name: 'john' }
User.parse({ id: '1', name: 'john' }); // => Error
```

### Combining Schemas <a name="combining-schemas"></a>
If you want to declare part of a schema that will be used elsewhere, you can import the `TJetSchema` type and use it to setup a partial schema, then merge it with your full schema later:
```typescript
import schema, { TJetSchema } from 'jet-schema';
import { isNumber, isString, isBoolean } from './validators';

const PartOfASchema: TJetSchema<{ id: number, name: string }> = {
 id: isNumber,
 name: isString,
} as const;

const FullSchema = schema<{ id: number, name: string, e: boolean }>({
 ...PartOfASchema,
 e: isBoolean,
});

console.log(FullSchema.new());
```

### TypeScript Caveats <a name="typescript-caveats"></a>
Due to how structural-typing works in typescript, there are some limitations with typesafety that you need to be aware of. To put things in perspective, if type `A` has all the properties of type `B`, we can use type `A` for places where type `B` is required, even if `A` has additional properties.

#### ▸ Validator functions
If an object property's type can be `string | undefined`, then a validator-function whose type-predicate only returns `param is string` will still work. However a if a type predicate returns `param is string | undefined` we cannot use it for type `string`. This could cause runtime issues if a you pass a validator function like `isString` (when you should have passed `isOptionalString`) to a property whose value ends up being `undefined`:
```typescript
interface IUser {
  id: string;
  name?: string;
}

const User = schema<IUser>({
 id: isString, // "isOptionalString" will throw type errors
 name: isOptionalString, // "isString" will not throw type errors but will throw runtime errors
});
```

#### ▸ Child schemas
As mentioned, if a property in a parent-schema is a mapped-object type (it has a defined set of keys), then you need to call `schema` again for the nested object. If you don't use a generic on the child-schema, typescript will still make sure all the required properties are there; however, because of structural-typing the child could have additional properties. It is highly-recommended that you pass a generic to your child-objects so additional properties don't get added:
```typescript
interface IUser {
  id: number;
  address?: { street: string } | null;
}

const User = schema<IUser>({
 id: isNumber,
 address: schema<IUser['address']>({
 street: isString,
 // foo: isString, // If we left off the generic <IUser['address']> we could add "foo"
 }, { nullish: true }),
});
```
> If you know of a way to enforce typesafety on child-object without requiring a generic please make a pull-request because I couldn't figure out a way.

### Bonus Features <a name="bonus-features"></a>
- When passing the `Date` constructor, `jet-schema` sets the type to be a `Date` object and automatically converts all valid date values (i.e. `string/number`, maybe a `Date` object got stringified in an API call) to a `Date` object. The default value will be a `Date` object with the current datetime. 
- You can also use an `enum` as a validator. The default value will be the first value in the enum object and validation will make sure it is value of that enum. **IMPORTANT** this does not work for mixed enums see: `eslint@typescript-eslint/no-mixed-enums`

### Using jet-schema without TypeScript <a name="without-typescript"></a>
`jet-schema` is built in TypeScript for TypScript but can be used directly with plain JavaScript. There are two minified files you can import if you want to use plain javascript:
 - `dist/index.min.js`: CommonJS
 - `dist/index.min.mjs`: ESM (es6)

## Tips <a name="tips"></a>

### Creating wrapper functions <a name="creating-wrapper-functions"></a>
If you need to modify the value of the `.test` function for a property, (like removing `nullables`) then I recommended merging your schema with a new object and adding a wrapper function around that property's test function.
```typescript
// models/User.ts
import { nonNullable } from 'util/validators.ts';

interface IUser {
  id: number;
  address?: { street: string, zip: number } | null;
}

const User = schema<IUser>({
 id: isNumber,
 address: schema<IUser['address']>({
 street: isString,
 zip: isNumber,
 }, { nullish: true }),
});

export default {
  // Wrapper function to remove nullables
  checkAddr: nonNullable(User.pick('address').test),
  ...User,
} as const;
```

### Recommended Globals Settings <a name="recommended-global-settings"></a>
I highly recommend you set these default values for each of your basic primitive validator-functions, unless of course your application has some other specific need:
```typescript
import { isNumber, isString, isBoolean } from 'util/validators.ts';

export default jetSchema({
  globals: [
    { vf: isNumber, default: 0 },
    { vf: isString, default: '' },
    { vf: isBoolean, default: false },
  ],
});
```

### jet-validators <a name="jet-validators"></a>
<a href="https://github.com/seanpmaxwell/jet-validators">jet-validators</a> is a library which contains a long list of the most commonly needed validator-functions. If you have a large application (such as an enterprise website), I recommended installing **jet-validators** as well to avoid having to define some of the more generic validators.

```typescript
import { isNumber, isString, isBoolean } from 'jet-validators';

// Your custom validator
const isRelationalKey = (arg: unknown): arg is number => {
  return isNumber(arg) && arg >= -1;
};

const schema = jetSchema({
  globals: [
    { vf: isNumber, default: 0 },
    { vf: isString, default: '' },
    { vf: isBoolean, default: false },
    { vf: isRelationalKey, default: -1 },
  ],
});

const User = schema({
  id: isRelationalKey,
  name: isString,
});
```

> If you're wondering when to use jet-schema vs `parseObject` from jet-validators, a good rule of thumb is to use jet-schema for any objects that you are creating multiple instances of which may have predefined types (i.e. an object which represents a database table). For simple objects which just need a few fields validated and you aren't created multiple instances of them or applying types to them, just use `parseObject`.

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