The Performance Impact of Mutating Object Prototypes

In JavaScript, objects inherit properties and methods from their prototypes. While modifying these prototypes can seem straightforward, assigning to the proto property, or mutating the prototype chain after its creation, is highly discouraged due to its severe performance implications.

Modern JavaScript engines optimize property accesses based on the internal type of an object and its prototype chain. Modifying the prototype chain invalidates these optimizations, resulting in slower execution times.

Unlike assigning to individual properties on the prototype, such as Foo.prototype.bar, reassigning the proto property itself can have catastrophic effects on performance. This operation forces the engine to:

• Check the entire prototype chain for cycles
• Flush property lookup optimizations
• Discard precompiled code
• Fall back to slower, unoptimized code

Why the Warning:

The warning "very slow and unavoidably slows down subsequent execution" explicitly refers to these performance penalties associated with mutating the proto property. Changing the prototype chain compromises the ability of the engine to efficiently access and manage properties within that object.

Alternatives to Mutating Prototypes:

To avoid the performance issues associated with prototype mutation, consider these alternatives:

• Create new objects with the desired prototype chain using Object.create()
• Assign properties directly to the object's prototype using Foo.prototype.bar = bar