Force JavaScript exception/error when reading an undefined object property?

This can be achieved using ES6 proxies:

function disallowUndefinedProperties(obj) {
    const handler = {
        get(target, property) {
            if (property in target) {
                return target[property];
            }

            throw new Error(`Property '${property}' is not defined`);
        }
    };

    return new Proxy(obj, handler);
}

// example
const obj = { key: 'value' };
const noUndefObj = disallowUndefinedProperties(obj);

console.log(noUndefObj.key);
console.log(noUndefObj.undefinedProperty); // throws exception

This looks to me like a classic case of trying to shoehorn one language into the paradigms of another - better IMHO to change your coding style to follow how Javascript does things than try to make it conform to C++ concepts and expectations.

That said, if you want to throw an error as you suggest, you'll need to define some sort of custom getProperty function, either on the object you're trying to access or in the global scope. An implementation might look like this:

function getProperty(o, prop) {
    if (o.hasOwnProperty(prop)) return o[prop];
    else throw new ReferenceError('The property ' + prop + 
        ' is not defined on this object');
}

var o = {
    foo: 1,
    bar: false,
    baz: undefined
};

getProperty(o, 'foo'); // 1
getProperty(o, 'bar'); // false
getProperty(o, 'baz'); // undefined
getProperty(o, 'foobar'); 
// ReferenceError: The property baz is not defined on this object

But this is ugly, and now you've got this custom language construct in all of your code, making it less portable (if, for example, you wanted to copy any part of your code into another script, you'd have to copy your new function too) and less legible to other programmers. So I'd really recommend working within the Javascript paradigm and checking for undefined before accessing the properties you need (or setting up your code so that false-y values are expected and don't break things).

As to your second question, why Javascript throws an error for undefined variables but not for undefined object properties, I can't give any better answer than "Because that's what's in the language specification." Objects return undefined for undefined property names, but undefined variable references throw an error.

Is there any way to force an error or exception to be thrown when I read an undefined property?

That's possible using ES6 proxies as has been said in previous responses. I have done the small node module "zealit" to avoid having to implement it every time.

If someone is interested : https://www.npmjs.com/package/zealit

const zealit = require('zealit')

const ref = { foo: true, bar: undefined }
ref.foo // true 
ref.bar // undefined 
ref.baz // undefined 

const zealed = zealit(ref)
zealed.foo // true 
zealed.bar // undefined 
zealed.baz // throws a ReferenceError

Force JavaScript exception/error when reading an undefined object property?

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