typecheck-decorator

Lightweight runtime typechecking with nice syntax. Can be smoothly enabled in dev and disabled in prod.

Example

import T, { typecheck } from 'typecheck-decorator';

class Person {
  constructor(age, country, city, zipCode) {
    this.age = age;
    this.country = country;
    this.city = city;
    this.zipCode = zipCode;
  }

  // takes a number and returns a number
  @typecheck([T.Number()], T.Number())
  multiplyAgeBy(n) {
    return this.age * n;
  }

  // takes an object describing the country and the city, and an
  // optional zipcode, and returns a bool.
  // the city can be either 'Paris' or 'New York'
  // the country can be either 'France' or 'United States'
  // the zipcode must be correctly formatted
  @typecheck(
    [
      T.shape({
        country: T.oneOf(T.exactly('France'), T.exactly('United States')),
        city: T.oneOf(T.exactly('Paris'), T.exactly('New York')),
      }),
      T.option(T.String({ match: /^[0-9]{5}$/ })),
    ],
    T.bool()
  )
  livesIn({ country, city }, zipCode) {
    return
      this.country === country &&
      this.city === city &&
      zipCode && this.zipCode === zipCode;
  }

  // alternate syntax, to ignore return type and remove []
  @takes(
    T.Number(),
    T.eachOf(
      T.String(),
      T.oneOf(T.exactly('Paris'), T.exactly('New York'))
    )
  )
  setAgeAndCountry(age, country) {
    this.age = age;
    this.country = country;
  }

  // alternate syntax, to ignore args types
  @returns(T.Number())
  getAge() {
    return this.age;
  }

  // combine alternate syntax
  @takes(T.shape({
    foo: T.String(),
    buzz: T.String(),
  }))
  @returns(T.shape({
    bar: T.String(),
    buzz: T.String(),
  }))
  static reverse({ foo, bar }) {
    return {
      [foo]: 'foo',
      [bar]: 'bar',
    };
  }
}

// also works with regular functions
// expects two numbers and returns their sum
const sum = typecheck([T.Number(), T.Number()], T.Number(),
  (a, b) => a + b,
);

API

typecheck(argsTypes = [], valueType = T.any(), fn) returns fn wrapped with type checks.

@typecheck(argsTypes = [], valueType = T.any()) decorates a method with type checks.

@takes(...argsTypes) is an alias for @typecheck(argTypes, void 0) which ignores return value type.

@returns(valType) is an alias for @typecheck(void 0, valType) which ignores arguments types.

Custom types

Types can be described using the provided type descriptors and operators. A type being basically an assertion, you can easily implement your own:

T.zipCode = (x) => assert(x.match(/^[0-9]{5}$/) !== null);

Type operators return new types which can be assigned to variables or reused in expressions:

const optZipCode = T.option(T.zipCode);
const nullableStringOrOptZipCode = T.oneOf(optZipCode, T.nullable(T.String()))

See the example tests at the end of this file to have an idea of what type descriptors and operators are provided.

Disabling typechecking

While runtime typecheck is immensely useful during development, some of it is often best left out of the production builds. On the other hand you may want to leave some typechecking active even in production, eg. for user input typechecking.

The recommended way to do this is to pair typecheck-decorator with babel-plugin-strip-decorator. For example you can strip all decorators referred to as devTypecheck while keeping all decorators referred to as prodTypecheck, like the following:

import { typecheck } from 'typecheck-decorator';
const devTypecheck = prodTypecheck = typecheck;

class A {
  @devTypecheck(
    [T.Number(), T.Number()],
    T.Number()
  ) // will be stripped away
  static sum(a, b) {
    return a + b;
  }

  @prodTypecheck(
    [T.Number(), T.Number()],
    T.Number()    
  ) // will NOT be stripped
  static untrustedSum(a, b) {
    return a + b;
  }
}

import T, { typecheck, takes, returns } from 'typecheck-decorator';

describe('T', () => {
  it('T.any()', () => {
    should(() => T.any()(42)).not.throw();
    should(() => T.any()(void 0)).not.throw();
    should(() => T.any()(null)).not.throw();
    should(() => T.any()({})).not.throw();
  });
  it('T.instanceOf()', () => {
    class C1 {}
    class C2 {}
    const a = new C1();
    const b = new C2();
    should(() => T.instanceOf(C1)(a)).not.throw();
    should(() => T.instanceOf(C1)(b)).throw();
    should(() => T.instanceOf(C2)(a)).throw();
    should(() => T.instanceOf(C2)(b)).not.throw();
  });
  it('T.exactly()', () => {
    should(() => T.exactly(42)(42)).not.throw();
    should(() => T.exactly(42)(1337)).throw();
    should(() => T.exactly({})({})).throw();
    should(() => T.exactly(JSON.stringify({}))(JSON.stringify({}))).not.throw();
  });
  it('T.deepEqual()', () => {
    should(() => T.deepEqual({ foo: 'bar' })({ foo: 'baz' })).throw();
    should(() => T.deepEqual({ foo: 'bar' })({ foo: 'bar' })).not.throw();
    should(() => T.deepEqual({ foo: { bar: 'fizz' } })({ foo: { bar: 'fizz' } })).not.throw();
  });
  it('T.bool()', () => {
    should(() => T.bool()(42)).throw();
    should(() => T.bool()({})).throw();
    should(() => T.bool()(true)).not.throw();
    should(() => T.bool()(false)).not.throw();
  });
  it('T.Number()', () => {
    should(() => T.Number()(42)).not.throw();
    should(() => T.Number()('42')).throw();
    should(() => T.Number({ above: 0 })(42)).not.throw();
    should(() => T.Number({ above: 0 })(-1)).throw();
    should(() => T.Number({ below: 0 })(0)).throw();
    should(() => T.Number({ below: 0 })(-1)).not.throw();
    should(() => T.Number({ within: [0, 1] })(0.5)).not.throw();
    should(() => T.Number({ within: [2, 3] })(0.5)).throw();
  });
  it('T.String()', () => {
    should(() => T.String()(42)).throw();
    should(() => T.String()('42')).not.throw();
    should(() => T.String({ length: 4 })('abc')).throw();
    should(() => T.String({ length: 4 })('abcd')).not.throw();
    should(() => T.String({ match: /^[0-9]*$/ })('abc')).throw();
    should(() => T.String({ match: /^[0-9]*$/ })('42')).not.throw();
  });
  it('T.Array()', () => {
    should(() => T.Array()(42)).throw();
    should(() => T.Array()([])).not.throw();
    should(() => T.Array()({})).throw();
    should(() => T.Array({ type: T.Number() })([42])).not.throw();
    should(() => T.Array({ type: T.Number() })([42, '42'])).throw();
    should(() => T.Array({ length: 0 })([])).not.throw();
    should(() => T.Array({ length: 0 })([42])).throw();
  });
  it('T.Object()', () => {
    should(() => T.Object()({})).not.throw();
    should(() => T.Object()(42)).throw();
    should(() => T.Object({ type: T.Number() })({ a: 42 })).not.throw();
    should(() => T.Object({ type: T.Number() })({ a: 42, b: '42' })).throw();
    should(() => T.Object({ length: 0 })({})).not.throw();
    should(() => T.Object({ length: 42 })({})).throw();
  });
  it('T.Promise()', () => {
    should(() => T.Promise()({})).throw();
    should(() => T.Promise()(Promise.resolve())).not.throw();
    return Promise.all([
      T.Promise({ type: T.Number() })(Promise.resolve(42)),
      T.Promise({ type: T.not(T.Number()) })(Promise.resolve('42')),
    ]);
  });
  it('T.Error()', () => {
    should(() => T.Error()(new Error())).not.throw();
    should(() => T.Error()({ message: 'foo' })).throw();
    should(() => T.Error({ message: 'foo' })(new Error('foo'))).not.throw();
    should(() => T.Error({ message: 'foo' })(new Error('bar'))).throw();
  });
  it('T.eachOf()', () => {
    class A {}
    class B extends A {}
    class C extends B {}
    should(() => T.eachOf(T.Object(), T.Array())([])).not.throw();
    should(() => T.eachOf(T.Object(), T.Array())(42)).throw();
    should(() => T.eachOf(T.instanceOf(A), T.instanceOf(B), T.instanceOf(C))(new C())).not.throw();
  });
  it('T.oneOf()', () => {
    should(() => T.oneOf(T.Number(), T.Object())(42)).not.throw();
    should(() => T.oneOf(T.Array(), T.Object())(42)).throw();
  });
  it('T.not()', () => {
    should(() => T.not(T.Object())(42)).not.throw();
    should(() => T.not(T.Object())({})).throw();
  });
  it('T.nullable()', () => {
    should(() => T.nullable(T.Number())(null)).not.throw();
    should(() => T.nullable(T.Number())(42)).not.throw();
    should(() => T.nullable(T.Number())('42')).throw();
    should(() => T.nullable(T.Number())(void 0)).throw();
  });
  it('T.option()', () => {
    should(() => T.option(T.Number())(void 0)).not.throw();
    should(() => T.option(T.Number())(42)).not.throw();
    should(() => T.option(T.Number())('42')).throw();
    should(() => T.option(T.Number())(null)).throw();
  });
  it('T.shape()', () => {
    should(() => T.shape({})(42)).throw();
    should(() => T.shape({ a: T.Number() })({ a: 42 })).not.throw();
    should(() => T.shape({ a: T.Number() })({ a: '42' })).throw();
    should(() => T.shape({ foo: T.String(), fizz: T.exactly('buzz') })({ foo: 'bar', fizz: 'buzz' })).not.throw();
    should(() => T.shape([])(42)).throw();
    should(() => T.shape([T.Number()])([42])).not.throw();
    should(() => T.shape([T.Number()])(['42'])).throw();
  });
  it('T.toPropType()', () => {
    const propTypeNumber = T.toPropType(T.Number());
    should(propTypeNumber({ x: 1337 }, 'x')).not.be.an.Error();
    should(propTypeNumber({ x: '1337' }, 'x')).be.an.Error();
  });
});

describe('typecheck', () => {
  const shouldTypeCheck = T.shouldTypeCheck;
  before(() => T.shouldTypeCheck = true);
  after(() => T.shouldTypeCheck = shouldTypeCheck);
  it('regular function typecheck', () => {
    const sum = typecheck([T.Number(), T.Number()], T.Number(),
      (a, b) => a + b
    );
    should(() => sum(1, 2)).not.throw();
    should(() => sum('42', '42')).throw();
  });
  it('class static method typecheck', () => {
    class A {
      @typecheck([T.Number(), T.Number()], T.Number())
      static sum(a, b) {
        return a + b;
      }
    }
    should(() => A.sum(1, 2)).not.throw();
    should(() => A.sum('42', '42')).throw();
  });
  it('class method typecheck', () => {
    class A {
      constructor(a, b) {
        this.a = a;
        this.b = b;
      }
      @typecheck([], T.Number())
      sum() {
        return this.a + this. b;
      }
    }
    should(() => new A(1, 2).sum()).not.throw();
    should(() => new A('42', '42').sum()).throw();
  });
  it('class static method takes', () => {
    class A {
      @takes(T.Number(), T.Number())
      static sum(a, b) {
        return a + b;
      }
    }
    should(() => A.sum(1, 2)).not.throw();
    should(() => A.sum('42', '42')).throw();
  });
  it('class method takes', () => {
    class A {
      constructor(a, b) {
        this.a = a;
        this.b = b;
      }
      @takes(T.Number())
      sum(c) {
        return this.a + this.b + c;
      }
    }
    should(() => new A(1, 2).sum(3)).not.throw();
    should(() => new A('42', '42').sum('42')).throw();
  });
  it('class static method returns', () => {
    class A {
      @returns(T.Number())
      static sum(a, b) {
        return a + b;
      }
    }
    should(() => A.sum(1, 2)).not.throw();
    should(() => A.sum('42', '42')).throw();
  });
  it('class method returns', () => {
    class A {
      constructor(a, b) {
        this.a = a;
        this.b = b;
      }
      @returns(T.Number())
      sum() {
        return this.a + this.b;
      }
    }
    should(() => new A(1, 2).sum()).not.throw();
    should(() => new A('42', '42').sum()).throw();
  });
  it('class static method takes & returns', () => {
    class A {
      @takes(T.Number(), T.any())
      @returns(T.Number())
      static sum(a, b) {
        return a + b;
      }
    }
    should(() => A.sum(1, 2)).not.throw();
    should(() => A.sum(42, '42')).throw();
  });
});