JS变量的默认参数值:为什么标识符必须不同? [英] JS default argument value from variable: why must identifier be different?

问题描述

使用具有相同名称的变量分配默认值会引发引用错误:

var a = 'adef';
var x = (a=a) => console.log(a);
x();
=> "ReferenceError: a is not defined"

但这很好:

var other = 'otherdef';
var x = (a=other) => console.log(a);
x();
=> "otherdef"

我的假设是将外部范围中的a的值分配给新范围.

我尝试使用const代替var，并使用class/function代替箭头功能，但结果始终相同(在chrome 63和node 6中测试). >

我感觉到问题是a在赋值期间被吊起"，因此赋值是指新的"a"(存在但未定义)...

解决方案

此行为的目的是允许将一个参数默认初始化为另一个参数的值，例如:

var a = 2;
var x = (a, b = a) => console.log(a, b);
x(42); // 42 42

使特例a = a的工作方式有所不同 可以完成，但这将使重构使用此行为的函数更加困难(如果没有此功能，您将无法重命名参数a还重命名它所依赖的变量.

Assigning a default value using a variable with the same name throw a reference error:

var a = 'adef';
var x = (a=a) => console.log(a);
x();
=> "ReferenceError: a is not defined"

But this is fine:

var other = 'otherdef';
var x = (a=other) => console.log(a);
x();
=> "otherdef"

My assumption was that the value of a in the outer scope would be assigned to the new scope.

I have tried using const instead of var, and class/function instead of an arrow-function, but the result is always the same (tested in chrome 63 and node 6).

I have a feeling the issue is that a is 'hoisted' during assignment and so the assignment is referring to the new 'a' (which exists but is undefined)...

解决方案

The purpose of this behavior is to allow a parameter to be default-initialized to the value of another parameter, e.g:

var a = 2;
var x = (a, b = a) => console.log(a, b);
x(42); // 42 42

Making the special case a = a work differently could be done but that would make it harder to refactor functions that use this behavior (you wouldn't be able to rename the parameter a without also renaming the variable that it depends on).

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