为什么函数默认返回`undefined`而不是`null`？ [英] Why do functions return `undefined` instead of `null` by default?

问题描述

为什么JavaScript函数默认返回 undefined 而不是 null ？这是规范完全武断的选择，还是有一个更大的ECMAScript行为上下文，可以理解这个特定的选择？

Why do JavaScript functions return undefined by default instead of null? Is this a totally arbitrary choice by the specification, or is there a larger ECMAScript-behavior context in which this particular choice can be understood?

function a() {}
a();
// undefined

null之间有什么区别和 undefined ？是否存在基于规范的原因 undefined 更适合作为默认返回值，还是任意选择？

What is the difference between null and undefined? Is there a specification-based reason why is undefined more appropriate as a default return value, or was it an arbitrary choice?

推荐答案

规范中提到 null 和 undefined ：

The specification says of null and undefined:

未定义值

未为变量赋值时使用的原始值

primitive value used when a variable has not been assigned a value

空值

表示故意缺少任何对象值的原始值

primitive value that represents the intentional absence of any object value

undefined 表示无法分配值。这是一个值的完全缺席。 null 表示对象上下文中非值的肯定断言。 null 旨在在预期对象但当前值为无对象时使用。

undefined represents a failure to assign a value. It is the total absence of a value. null represents the positive assertion of a non-value in an object context. null is intended to be used when an object is expected but the current value is no-object.

鉴于这两个定义，很明显 undefined 是正确的选择，因为

Given these two definitions, it seems obvious that undefined is the correct choice, since

1. 函数可以返回除对象以外的值，并且


2. 未能指定返回值整齐地映射到无法分配值

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