A run-time type checker with proxies

typeCheck.js

function typeCheck(obj, definition) {
  return new Proxy(obj, {
    set(obj, key, value) {
      if (key in definition && typeof value !== definition[key]) {
        throw new TypeError(`Invalid type of ${key} '${typeof value}'. expected to be '${definition[key]}'`)
      }
      
      return Reflect.set(obj, key, value);
    }
  });
}

const userShape = {
  name: 'string',
  age: 'number'
}

class TypeSafe{
  constructor(shape){
    return typeCheck(this, shape);
  }
}

class User extends TypeSafe{
  constructor(name, age){
    super(userShape);
    this.name = name;
    this.age = age;
  }
}

let myUser = new User('sagiv', '36'); // throws -> TypeError: Invalid type of age 'string'. expected to be 'number'

typeCheck_nestedObj.js

// support nested objects
function typeCheck(obj, definition) {
  const validator = {
    set(obj, key, value) {
      if (key in definition) {

        if (Object.prototype.toString.call(value) === "[object Object]") {
          // this is an object
          Object.entries(value).forEach(([oKey, oVal]) => {
            // proxy the set of each key
            const nested = typeCheck(value, definition[key]);
            Reflect.set(nested, oKey, oVal);
          });
        } else if (typeof value !== definition[key]) {
          throw new TypeError(`Invalid type of ${key} '${typeof value}'. expected to be '${definition[key]}'`)
        }
      }

      return Reflect.set(obj, key, value);
    }
  }
  return new Proxy(obj, validator);
}

const userShape = {
  name: 'string',
  age: 'number',
  address: {
    where: 'string'
  }
}

class TypeSafe {
  constructor(shape) {
    return typeCheck(this, shape);
  }
}

class User extends TypeSafe {
  constructor(name, age, address) {
    super(userShape);
    this.name = name;
    this.age = age;
    this.address = address;
  }
}

let myUser = new User('sagiv', 36, {
  where: 0
}); // throws -> TypeError: Invalid type of where 'number'. expected to be 'string'

TODO: add complex objects support