Underscore.js v1.9.0 restArguments用switch处理startIndex的原因及范围疑问

Great question! This is a perfect example of the micro-optimizations utility libraries like Underscore.js use to balance performance and maintainability. Let’s break down your two questions clearly:

1. Is handling startIndex 0,1,2 separately to avoid unnecessary small loops? #

Absolutely. These are the most common real-world use cases for restArguments. Think about how often you write functions that take 0, 1, or 2 fixed parameters followed by a variable number of rest arguments.

For these small values, even a tiny loop introduces avoidable overhead: creating an empty array, incrementing an index counter, checking loop conditions, and pushing items one by one. By hardcoding these cases, Underscore skips all that extra work entirely—leading to faster execution for the code paths that get used most often. It’s a classic "optimize the hot paths" strategy.

2. Why only these three cases instead of expanding to more values? #

It’s all about diminishing returns and code maintainability.

As you go beyond startIndex = 2, the number of hardcoded branches you’d need to add grows quickly. For example, handling startIndex = 3 would require another switch case with logic to capture 3 fixed args plus the rest, and so on. At a certain point, the tiny performance gain from avoiding a loop is far outweighed by the cost of making the codebase bloated, repetitive, and harder to debug or update.

Underscore’s maintainers likely settled on 0,1,2 because these are the values that show up most frequently in practice. The optimization delivers meaningful performance benefits where it matters, without sacrificing the readability and maintainability of the code.

内容的提问来源于stack exchange，提问作者orangespark