Since you stated you're dealing with JavaScript, I'll offer 'assign' as a handy verb. In most programming languages, you need to declare an object property before assigning it, but in JavaScript the assignment operator ('=') both updates and creates properties (when they don't exist).
So if you have your generic object here, and you want to create/update properties of that object using a single function call, you're essentially assigning values to the properties of the object. Creation is implied.
myObject.prototype.assignProperties = (property, value) => this.property = value;
The only activity carried out in the function is 'assign' by the assignment operator. If the property doesn't exist, it is created.
Source: I'm a software engineer working with JavaScript regularly (FireFox OS applications).