Set default value of Javascript object attributes

Since I asked the question several years ago things have progressed nicely.

Proxies are part of ES6. The following example works in Chrome, Firefox, Safari and Edge:

let handler = {
  get: function(target, name) {
    return target.hasOwnProperty(name) ? target[name] : 42;
  }
};

let emptyObj = {};
let p = new Proxy(emptyObj, handler);

p.answerToTheUltimateQuestionOfLife; //=> 42

Read more in Mozilla's documentation on Proxies.

Use destructuring (new in ES6)

There is great documentation by Mozila as well as a fantastic blog post that explains the syntax better than I can.

To Answer Your Question

var emptyObj = {};
const { nonExistingAttribute = defaultValue } = emptyObj;
console.log(nonExistingAttribute); // defaultValue

Going Further

Can I rename this variable? Sure!

const { nonExistingAttribute: coolerName = 15} = emptyObj;
console.log(coolerName); // 15

What about nested data? Bring it on!

var nestedData = {
    name: 'Awesome Programmer',
    languages: [
        {
            name: 'javascript',
            proficiency: 4,
        }
    ],
    country: 'Canada',
};

var {name: realName, languages: [{name: languageName}]} = nestedData ;

console.log(realName); // Awesome Programmer
console.log(languageName); // javascript

There isn't a way to set this in Javascript - returning undefined for non-existent properties is a part of the core Javascript spec. See the discussion for this similar question. As I suggested there, one approach (though I can't really recommend it) would be to define a global getProperty function:

function getProperty(o, prop) {
    if (o[prop] !== undefined) return o[prop];
    else return "my default";
}

var o = {
    foo: 1
};

getProperty(o, 'foo'); // 1
getProperty(o, 'bar'); // "my default"

But this would lead to a bunch of non-standard code that would be difficult for others to read, and it might have unintended consequences in areas where you'd expect or want an undefined value. Better to just check as you go:

var someVar = o.someVar || "my default";