Why Is It Unwise to Assign Prototype Methods Within Constructor Functions?

Assigning Prototype Methods Within the Constructor Function: Unwise Practice

When creating prototype objects, it's a common practice to assign methods within the constructor function itself. However, this approach raises concerns about drawbacks and unexpected scoping issues.

Drawbacks:

• Repeated Prototype Assignment: With each instance creation, the prototype is reassigned to a locally defined function, repeating the assignment and creating a new function object each time. This increases runtime cost and garbage collection overhead compared to assigning the prototype outside the constructor.
• Scope Issues: When referencing local constructor variables from the assigned prototype method, unexpected scoping issues can arise. In some scenarios, this can lead to a bug where one instance's method accesses the local variables of another.

Runtime Consequences:

In terms of performance, assigning methods directly on the object (as opposed to the prototype) offers improved execution speed. This is because accessing object methods directly bypasses the prototype lookup.

Scoping Caveat:

The practice of assigning prototype methods within the constructor can be particularly problematic in scenarios where multiple instances of the object exist. As seen in the provided Counter example, each instance's get method references the same local variables of the constructor of the last instance created. This results in unexpected and incorrect behavior.

Recommendation:

To avoid these issues and ensure reliable code, it's advisable to assign prototype methods outside of the constructor function. This promotes clarity, avoids scoping problems, and enhances performance.

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