在Javascript中使用单个元素的数组上调用Array.reduce [英] Calling Array.reduce on an array with a single element in Javascript

问题描述

在空数组上调用reduce会抛出 TypeError ，这是完全可以理解的，并且有助于捕获错误。但是，当我在其中具有单个项的数组上调用它时，该行为使我​​感到困惑：

Calling reduce on an empty array throws TypeError which is perfectly understandable and helps catching bugs. But when I call it on an array with a single item inside, the behavior confuses me:

var arr = ["a"];
arr.reduce(function(a,b){
   return [a,b]
}); //returns "a"

我知道reduce并不意味着要在这样的数组上使用，但是我发现仅返回元素而不调用回调或引发错误至少很奇怪。

I know that reduce is not meant to be used on such an array, but I find that returning just the element without invoking the callback or throwing an error is at least strange.

此外，MDN文档指出，回调是对数组中的每个值执行四个参数：。

Furthermore, the MDN documentation states that the callback is a "Function to execute on each value in the array, taking four arguments:".

有人可以解释此行为背后的原因吗？

Can someone explain the reasoning behind this behaviour?

推荐答案

该回调应该是二进制函数（即，该函数需要两个参数进行操作，另外两个参数还包含 currentIndex 和原始的 array ）。

The callback is supposed to be a "binary function" (i.e. one that takes two arguments to operate on, plus the additional two arguments that hold the currentIndex and the original array).

如果仅提供一个元素，则将 currentValue 或 previousValue 的未定义值传递给回调，可能导致运行时错误。

If only one element is supplied, you would be passing an undefined value to the callback for either currentValue or previousValue, potentially causing runtime errors.

因此设计假定仅给定一个值（wh以太是一个空数组，一个 initialValue 或一个只有一个值且没有 initialValue 的数组）最好不要完全调用回调。

The design therefore assumes that given only one value (whether that be an empty array, and an initialValue, or an array with one value and no initialValue) it's better not to invoke the callback at all.

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