Set default value of Javascript object attributes

Is there a way to set the default attribute of a Javascript object such that:

let emptyObj = {};
// do some magic
emptyObj.nonExistingAttribute // => defaultValue

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";

my code is:

function(s){
    s = {
        top: s.top || 100,    // default value or s.top
        left: s.left || 300,  // default value or s.left
    }
    alert(s.top)
}

The way I achieve this is with the object.assign function

const defaultProperties = { 'foo': 'bar', 'bar': 'foo' };
const overwriteProperties = { 'foo': 'foo' };
const newObj = Object.assign({}, defaultProperties, overwriteProperties);
console.log(defaultProperties);  // {"foo": "bar", "bar": "foo"}
console.log(overwriteProperties);  // { "foo": "foo" };
console.log(newObj);  // { "foo": "foo", "bar": "foo" }

This sure sounds like the typical use of protoype-based objects:

// define a new type of object
var foo = function() {};  

// define a default attribute and value that all objects of this type will have
foo.prototype.attribute1 = "defaultValue1";  

// create a new object of my type
var emptyObj = new foo();
console.log(emptyObj.attribute1);		// outputs defaultValue1

This seems to me the most simple and readable way of doing so:

let options = {name:"James"}
const default_options = {name:"John", surname:"Doe"}

options = Object.assign({}, default_options, options)

Object.assign() reference

Or you can try this

dict = {
 'somekey': 'somevalue'
};

val = dict['anotherkey'] || 'anotherval';

I think the simplest approach is using Object.assign.

If you have this Class:

class MyHelper {
	constructor(options) {
		this.options = Object.assign({
			name: "John",
			surname: "Doe",
			birthDate: "1980-08-08"
		}, options);
	}
}

You can use it like this:

let helper = new MyHelper({ name: "Mark" });
console.log(helper.options.surname); // this will output "Doe"

Documentation (with polyfill): https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

Simplest of all Solutions:

dict = {'first': 1,
	    'second': 2,
        'third': 3}

Now,

dict['last'] || 'Excluded'

will return 'Excluded', which is the default value.

I saw an article yesterday that mentions an Object.__noSuchMethod__ property: JavascriptTips I've not had a chance to play around with it, so I don't know about browser support, but maybe you could use that in some way?

I'm surprised nobody has mentioned ternary operator yet.

var emptyObj = {a:'123', b:'234', c:0};
var defaultValue = 'defaultValue';
var attr = 'someNonExistAttribute';
emptyObj.hasOwnProperty(attr) ? emptyObj[attr] : defaultValue;//=> 'defaultValue'


attr = 'c'; // => 'c'
emptyObj.hasOwnProperty(attr) ? emptyObj[attr] : defaultValue; // => 0

In this way, even if the value of 'c' is 0, it will still get the correct value.

var obj = {
  a: 2,
  b: 4
};

console.log(obj);

--> {a: 2, b: 4}

function applyDefaults(obj) {
  obj.a ||= 10;
  obj.b ||= 10;
  obj.c ||= 10;
}

// do some magic
applyDefaults(obj);

console.log(obj);

--> {a: 2, b: 4, c: 10}

This works because

undefined || "1111111" --> "1111111"
"0000000" || "1111111" --> "0000000"

as null, undefined, NaN, 0, "" (Empty String), false itself, are all considered to be equivalent to false (falsy). Anything else is true (truthy).

Note that this is not uniformly supported across browsers and nodejs versions (confirm for yourself).

So two troublesome cases are the empty String "" and 0 (zero). If it is important not to override those, you might need to rewrite this as:

if (typeof obj.d == "undefined") obj.d = "default"

This will be better supported across browsers also.

Alternatively you could write this as:

obj.d ??= "default"

This is the nullish assignment which applies only to values that are null or undefined (nullish) - of which the empty string is not part. However, this has again a diminished cross-browser support.

See also on the official Mozilla Website - Assigning a default value to a variable.

This is actually possible to do with Object.create. It will not work for "non defined" properties. But for the ones that has been given a default value.

var defaults = {
    a: 'test1',
    b: 'test2'
};

Then when you create your properties object you do it with Object.create

properties = Object.create(defaults);

Now you will have two object where the first object is empty, but the prototype points to the defaults object. To test:

console.log('Unchanged', properties);
properties.a = 'updated';
console.log('Updated', properties);
console.log('Defaults', Object.getPrototypeOf(properties));

Object.withDefault = (defaultValue,o={}) => {
  return new Proxy(o, {
    get: (o, k) => (k in o) ? o[k] : defaultValue 
  });
}

o = Object.withDefault(42);
o.x  //=> 42

o.x = 10
o.x  //=> 10
o.xx //=> 42

One approach would be to take a defaults object and merge it with the target object. The target object would override values in the defaults object.

jQuery has the .extend() method that does this. jQuery is not needed however as there are vanilla JS implementations such as can be found here:

http://gomakethings.com/vanilla-javascript-version-of-jquery-extend/

If you only have an object that is a single level deep (nested object properties will not merge as expected since it directly destructures from the first level), you can use the following destructuring syntax:

const options = {
    somevar: 1234,
    admin: true
};

const defaults = {
    test: false,
    admin: false,
};

var mergedOptions = {...defaults, ...options};

Of which the output would be:

console.log(options);
// { somevar: 1234, admin: true }

console.log(mergedOptions);
// { test: false, admin: true, somevar: 1234 }

Or even formatted as a single statement (this is slightly unreadable though):

const options = {...{
    // Defaults
    test: false,
    admin: false,
}, ...{
    // Overrides
    somevar: 1234,
    admin: true
}};

I came here looking for a solution because the header matched my problem description but it isn't what i was looking for but i got a solution to my problem(I wanted to have a default value for an attribute which would be dynamic something like date).

let Blog = {
title  : String,
image  : String,
body   : String,
created: {type: Date, default: Date.now}
}

The above code was the solution for which i finally settled.