Great question! This behavior boils down to rounding mode selection and historical design tradeoffs that JavaScript inherited from earlier languages. Let's break it down clearly:

• There's no single "standard mathematical rounding" rule
  A lot of people assume the familiar "round half up" (四舍五入) is the only "standard" way, but mathematics actually defines multiple rounding modes. JavaScript uses what's called "round half towards positive infinity": when the fractional part is exactly 0.5, it picks the integer closer to positive infinity. For positive numbers like 1.5, that means rounding up to 2; for negative numbers like -1.5, that means rounding to -1 (since -1 is larger than -2 and closer to +∞).

• Inheritance from Java's design
  JavaScript was built quickly in 1995, and it borrowed heavily from Java's syntax and standard library behavior—including the rounding logic of Math.round(). Java had already adopted this rounding mode, so JavaScript followed suit to make it familiar to developers who already knew Java. This reduced the learning curve for early JS developers, which was critical for the language's rapid adoption.

• Simplicity and consistency of rules
  This rounding rule is straightforward to implement and explain: "if the decimal is 0.5 or higher, round up to the next integer" (where "up" means towards positive infinity, no matter the sign). Compare this to more complex modes like bankers rounding (round half to even), where 1.5 rounds to 2 but 2.5 rounds to 2, and -1.5 rounds to -2 but -2.5 rounds to -2. That rule is harder to teach and code, and for most common use cases, the simpler "round towards +∞" was deemed sufficient.

• Practical error distribution
  While bankers rounding minimizes cumulative bias in large datasets, the mode JS uses has its own perks. For scenarios with small numbers or where strict statistical neutrality isn't a priority, the consistent "round up on 0.5" behavior feels intuitive and avoids the confusion of alternating even/odd results.

If you need a different rounding mode (like bankers rounding), you can easily implement custom logic in JavaScript—but the default Math.round() sticks to this historically inherited, simple rule.

内容的提问来源于stack exchange，提问作者4ou4