Copying properties from one object to another with a condition

Question

Lazy-me is wondering if there a better way to copy the properties in one object (source) over to another object (destination) only if the properties exist in the latter? It does not necessarily have to be using Underscore.

For example,

_.mixin({
    assign: function (o, destination, source) {
        for (var property in source) {
            if (destination.hasOwnProperty(property)) {
                destination[property] = source[property];
            }
        }
        return destination;
    }
});

console.log( _().assign({ a: 1, b: 2, d: 3 }, { a: 4, c: 5 }) ) // a: 4, b: 2, d: 3

Answer 1

Use Object.assign(obj1, obj2); (if the properties exist in the latter) which is native in ES6 (no underscore.js is required).

The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object. More info here.

Example:

var o1 = { a: 1 };
var o2 = { b: 2 };
var o3 = { c: 3 };

var obj = Object.assign(o1, o2, o3);
console.log(obj);

Alternatively use undescore.js

_.extend(destination, *sources)

or

_.extendOwn(destination, *sources)

Detailated information can be found here: http://underscorejs.org/#extend