03-enum-basic.js

// 03 - A better enum

function makeEnum(values) {
    const handler = {
        set (obj, prop, value) {
            throw new TypeError('Enum is read only');
        },
        get (obj, prop) {
            if (!(prop in obj)) {
                throw new ReferenceError(`Unknown enum key "${prop}"`);
            }
            return Reflect.get(obj, prop);
        },
        deleteProperty (obj, prop) {
            throw new TypeError('Enum is read only');
        }
    };
    return new Proxy(values, handler);
}

const someValue = 3;


// using a plain object as enum
console.log('Object');
const myObj = {ONE: 1, TWO: 2};
console.log(myObj.ONE);       // 1 - ok
console.log(myObj.TWWO);      // undefined - typos can lead to silent errors
if (myObj.ONE = someValue) {  // this mistyped condition evaluates to true
    console.log(myObj.ONE);   // 3 - and changes our enum too
}
delete myObj.ONE;             // deleted


// using a freezed object can prevent by Object.freeze)
console.log('Freezed object');
const myFrObj = Object.freeze({ONE: 1, TWO: 2});
console.log(myFrObj.ONE);       // 1 - ok
console.log(myFrObj.TWWO);      // undefined - typos can lead to silent errors
if (myFrObj.ONE = someValue) {  // still evaluates to true
    console.log(myFrObj.ONE);   // 1 - but at least the modification doesn't happen
}
delete myFrObj.ONE;             // no deletion, but no error either


// using a proxy as enum
console.log('Proxy');
const MyEnum = makeEnum({ONE: 1, TWO: 2});
console.log(MyEnum.ONE);           // 1 - ok
try {
    console.log(MyEnum.TWWO);      // ReferenceError - typos catched immediately  
} catch(ex) {
    console.error(ex);
}
try {
    if (MyEnum.ONE = someValue) {  // TypeError - can't be modified, doesn't evaluate, catched immediately 
        console.log(MyEnum.ONE);   // (this line never executes)
    }
} catch(ex) {
    console.error(ex);
}
try {
    delete MyEnum.ONE;             // TypeError
} catch(ex) {
    console.error(ex);
}