What is a common mistake when defining a function without the new keyword?

When defining a function and calling it without the new keyword, one of the significant impacts is on the context of the 'this' keyword. In a typical function call (as opposed to a constructor call using the new keyword), 'this' does not refer to the instance of an object that is being created. Instead, it refers to the global object (such as the window object in a browser context or global in Node.js), unless the function is called in strict mode, in which case 'this' would be undefined.

This behavior is crucial to understand in JavaScript, as it highlights how different contexts can influence variable scopes and object properties. When the function is invoked as a method of an object, 'this' points to that object. However, when just called as a standalone function, 'this' defaults to the global object, leading to potential bugs and unexpected behaviors, particularly if the function is intended to operate on an object or instance.

Understanding this concept is key for effectively using functions, as it ensures you correctly manage the context of 'this' and avoid issues related to scope and data manipulation.