Why are my JavaScript object properties being overwritten by other instances?

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There is only one BaseObject instance from which all TestObjects inherit. Don’t use instances for creating prototype chains!

What you want is:

var TestObject = function(){
    BaseObject.call(this); // give this instance own properties from BaseObject
    // do something
}
TestObject.prototype = Object.create(BaseObject.prototype);

See JavaScript inheritance: Object.create vs new, Correct javascript inheritance and What is the reason to use the ‘new’ keyword at Derived.prototype = new Base for a detailed explanation of the problems with new. Also have a look at Crockford’s Prototypal inheritance – Issues with nested objects

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