如何将 javascript var 设置为未定义
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/5795936/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to set a javascript var as undefined
提问by AnApprentice
Given:
鉴于：
console.log(boo); this outputs undefined
Given:
鉴于：
var boo = 1;
console.log(boo); this outputs 1
After defining boo and setting to 1, how can I then reset boo, so that console.log outputs undefined?
定义 boo 并设置为 1 后，如何重置 boo，以便 console.log 输出未定义？
Thanks
谢谢
采纳答案by neebz
delete boo
delete boo
Don't use var boo = undefined. undefined is just a variable and if someone sets undefined = "hello"then you'll be getting hello everywhere :)
不要使用var boo = undefined. undefined 只是一个变量，如果有人设置，undefined = "hello"那么你会到处打招呼:)
EDIT:
编辑：
null wasn't same as undefined. removed that bit.
null 与 undefined 不同。删除了那一点。
回答by Gooseberry
Solution
解决方案
To reliably set a variable booto undefined, use a function with an empty returnexpression:
要可靠地将变量设置boo为undefined，请使用带有空return表达式的函数：
boo = (function () { return; })();
After executing this line of code, typeof(boo)evaluates to 'undefined', regardless of whether or not the undefinedglobal property has been set to another value. For example:
执行这行代码后，typeof(boo)计算结果为'undefined'，无论undefined全局属性是否已设置为另一个值。例如：
undefined = 'hello';
var boo = 1;
console.log(boo); // outputs '1'
boo = (function () { return; })();
console.log(boo); // outputs 'undefined'
console.log(undefined); // outputs 'hello'
EDITBut see also @Colin's simpler solution!
编辑但另请参阅@Colin 的更简单的解决方案！
Reference
参考
This behavior is standard as far back as ECMAScript 1. The relevant specification states in part:
这种行为早在 ECMAScript 1 时就已成为标准。相关规范部分说明：
Syntax
return[no LineTerminatorhere] Expression;Semantics
A
returnstatement causes a function to cease execution and return a value to the caller. If Expressionis omitted, the return value isundefined.
句法
return[此处没有LineTerminator]表达式；语义
一个
return语句导致函数停止执行，并返回一个值给调用者。如果省略Expression，则返回值为undefined。
To view the original specifications, refer to:
要查看原始规格，请参阅：
Appendix
附录
For completeness, I have appended a brief summary of alternate approaches to this problem, along with objections to these approaches, based on the answers and comments given by other responders.
为完整起见，我根据其他响应者的回答和评论，附上了解决此问题的替代方法的简短摘要，以及对这些方法的反对意见。
1. Assign undefinedto boo
1. 分配undefined给boo
boo = undefined; // not recommended
Although it is simpler to assign undefinedto boodirectly, undefinedis not a reserved word and could be replacedby an arbitrary value, such as a number or string.
虽然这是更简单的分配来undefined向boo直接，undefined不是保留字和可以被取代由一个任意值，诸如数字或字符串。
2. Delete boo
2. 删除 boo
delete boo; // not recommended
Deleting booremoves the definition of booentirely, rather than assigning it the value undefined, and even then only worksif boois a global property.
删除boo会boo完全删除 的定义，而不是为其分配 value undefined，即使如此，也仅在boo是全局属性时才有效。
回答by Colin
Use the voidoperator. It will evaluate it's expression and then return undefined. It's idiomatic to use void 0to assign a variable to undefined
使用void运算符。它将评估它的表达式，然后返回undefined。void 0给变量赋值是惯用的方式undefined
var boo = 1; // boo is 1
boo = void 0; // boo is now undefined
Learn more here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
在此处了解更多信息：https: //developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
回答by Bryan Kyle
You can simply assign a variable the value of undefined:
您可以简单地将未定义的值分配给变量：
boo = undefined;
Alternatively, you can use the deleteoperator to delete the variable:
或者，您可以使用delete运算符删除变量：
delete boo;
回答by lobster1234
This works on Chrome Javascript Console:
这适用于 Chrome Javascript 控制台：
delete(boo)
回答by McStretch
var boo = 1;
console.log(boo); // prints 1
boo = undefined;
console.log(boo); // now undefined
