与简单的属性相比，变量具有属性[[可配置的]]，这意味着不可能通过删除操作符删除变量。

但是，有一个执行上下文不受此规则的影响。它是eval上下文:其中没有为变量设置[[Configurable]]属性。

注意，delete操作成功时返回true。

2021年更新:在Chrome 88和Firefox 84上测试:

implicit_global = 1;
delete implicit_global; // true

window.explicit_global = 1;
delete explicit_global; // true

const _object = {property: 1};
delete _object.property; // true

function_set = function() {};
delete function_set; // true

function function_declaration() {};
delete function_declaration; // false

(function () {
    var _var = 1;
    console.log(delete _var); // false
    console.log(_var); // 1
})()

(function () {
    let _let = 1;
    console.log(delete _let); // false
    console.log(_let); // 1
})()

(function () {
    const _const = 1;
    console.log(delete _const); // false
    console.log(_const); // 1
})()

由于浏览器更新，此答案的先前编辑已不再相关。

delete操作符从对象中删除属性。

delete object.property
delete object['property']

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete

根据问题，你需要下列其中一项

delete some_var;
delete window.some_var;
delete window['some_var'];

如果隐式声明变量时不使用var，正确的方法是使用delete foo。

然而，在你删除它之后，如果你试图在一个操作中使用它，比如添加，一个ReferenceError将被抛出，因为你不能将一个字符串添加到一个未声明的，未定义的标识符。例子:

x = 5;
delete x
alert('foo' + x )
// ReferenceError: x is not defined

在某些情况下，将它赋值为false、null或undefined可能更安全，这样它就被声明了，不会抛出这种类型的错误。

foo = false

请注意，在ECMAScript中，null, false, undefined, 0, NaN或“都将计算为false。只要确保你不使用!==操作符，而是在类型检查布尔值时，你不想进行身份检查(因此null将== false和false == undefined)。

还要注意，delete并没有“删除”引用，而是直接删除对象上的属性，例如:

bah = {}, foo = {}; bah.ref = foo;

delete bah.ref;
alert( [bah.ref, foo ] )
// ,[object Object] (it deleted the property but not the reference to the other object)

如果你用var声明了一个变量，你不能删除它:

(function() {
    var x = 5;
    alert(delete x)
    // false
})();

在Rhino中:

js> var x
js> delete x
false

也不能删除一些预定义的属性，比如Math。PI:

js> delete Math.PI
false

就像任何语言一样，有一些奇怪的例外需要删除，如果你足够在乎，你应该读一下:

https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Operators/Special_Operators/delete_Operator http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf

TLDR:简单定义的变量(没有var, let, const)可以用delete删除。如果你使用var, let, const -它们不能被delete或Reflect.deleteProperty删除。

Chrome 55:

simpleVar = "1";
"1"
delete simpleVar;
true
simpleVar;
VM439:1 Uncaught ReferenceError: simpleVar is not defined
    at <anonymous>:1:1
(anonymous) @ VM439:1
var varVar = "1";
undefined
delete varVar;
false
varVar;
"1"
let letVar = "1";
undefined
delete letVar;
true
letVar;
"1"
const constVar="1";
undefined
delete constVar;
true
constVar;
"1"
Reflect.deleteProperty (window, "constVar");
true
constVar;
"1"
Reflect.deleteProperty (window, "varVar");
false
varVar;
"1"
Reflect.deleteProperty (window, "letVar");
true
letVar;
"1"

Firefox Nightly 53.0a1显示相同的行为。

ECMAScript 2015提供了Reflect API。可以使用Reflect.deleteProperty()删除对象属性:

Reflect.deleteProperty(myObject, 'myProp');
// it is equivalent to:
delete myObject.myProp;
delete myObject['myProp'];

删除全局窗口对象的一个属性:

Reflect.deleteProperty(window, 'some_var');

在某些情况下，属性不能被删除(当属性不可配置时)，然后该函数返回false(以及删除操作符)。在其他情况下，它返回true:

Object.defineProperty(window, 'some_var', {
    configurable: false,
    writable: true,
    enumerable: true,
    value: 'some_val'
});

var frozen = Object.freeze({ myProperty: 'myValue' });
var regular = { myProperty: 'myValue' };
var blank = {};

console.log(Reflect.deleteProperty(window, 'some_var')); // false
console.log(window.some_var); // some_var

console.log(Reflect.deleteProperty(frozen, 'myProperty')); // false
console.log(frozen.myProperty); // myValue

console.log(Reflect.deleteProperty(regular, 'myProperty')); // true
console.log(regular.myProperty); // undefined

console.log(Reflect.deleteProperty(blank, 'notExistingProperty')); // true
console.log(blank.notExistingProperty); // undefined

在严格模式下运行时，deleteProperty函数和delete操作符之间有区别:

'use strict'

var frozen = Object.freeze({ myProperty: 'myValue' });

Reflect.deleteProperty(frozen, 'myProperty'); // false
delete frozen.myProperty;
// TypeError: property "myProperty" is non-configurable and can't be deleted