Using Proxy API to define custom validation for property values

proxy-custom-validation.js

const stringValidatorHandler = {
	set: (obj, key, value) => {
  	if (typeof value !== 'string') {
    	throw new Error('Expected string!')
    }
    
    obj[key] = value.toUpperCase();
  }
}

const weirdObject = new Proxy({}, stringValidatorHandler);

weirdObject.a = 5 // Error: Expected string!
weirdObject.a = 'abacaba';
console.log(weirdObject.a); // ABACABA