javascript - 为什么原型(prototype)函数比默认声明的函数慢 40 倍？

Question

我玩过 jsperf.com，发现原型(prototype)函数比“默认”声明的函数慢 40 倍。

String.prototype.contains = function(s){ return !!~this.indexOf(s) } = 220K ops/s

对比

function isContains(str, s) { return !!~str.indexOf(s) } = 8.5KK 操作/秒

Here's a jsperf test case

附言我知道原型(prototype)修改不是最好的情况，可以命名为“猴子修补”:)

Answer 1

我认为它很慢，因为字符串基元每次调用一个方法时都会自动用一个临时对象包装。

这也解释了 new Object("hi").foo() 相对于 "hi".foo() 的性能提升。

来自MDN docs :

String literals (denoted by double or single quotes) and strings returned from String calls in a non-constructor context (i.e., without using the new keyword) are primitive strings. JavaScript automatically converts primitives and String objects, so that it's possible to use String object methods for primitive strings. In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup.

附近:

Why can't I add properties to a string object in javascript?

String object versus literal - modifying the prototype?