是否有标准函数来检查 JavaScript 中的 null、未定义或空白变量？

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

提问by Alex

Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not undefinedor null? I've got this code, but I'm not sure if it covers all cases:

是否有一个通用的 JavaScript 函数来检查变量是否具有值并确保它不是undefined或null？我有这个代码，但我不确定它是否涵盖所有情况：

function isEmpty(val){
    return (val === undefined || val == null || val.length <= 0) ? true : false;
}

回答by jAndy

You can just check if the variablehas a truthyvalue or not. That means

您可以只检查变量是否有truthy值。这意味着

if( value ) {
}

will evaluate to trueif valueis not:

将评估为trueif valueis not：

• null
• undefined
• NaN
• empty string ("")
• 0
• false

• 空值
• 不明确的
• NaN
• 空字符串 ("")
• 0
• 错误的

The above list represents all possible falsyvalues in ECMA-/Javascript. Find it in the specificationat the ToBooleansection.

上面的列表代表falsy了 ECMA-/Javascript 中所有可能的值。在部分的规范中找到它ToBoolean。

Furthermore, if you do not knowwhether a variable exists (that means, if it was declared) you should check with the typeofoperator. For instance

此外，如果您不知道变量是否存在（也就是说，如果已声明），您应该检查typeof操作符。例如

if( typeof foo !== 'undefined' ) {
    // foo could get resolved and it's defined
}

If you can be sure that a variableis declared at least, you should directly check if it has a truthyvalue like shown above.

如果您可以确定至少声明了一个变量，则应直接检查它是否具有truthy如上所示的值。

Further read: http://typeofnan.blogspot.com/2011/01/typeof-is-fast.html

进一步阅读：http: //typeofnan.blogspot.com/2011/01/typeof-is-fast.html

回答by Salman A

The verbose method to check if value is undefined or null is:

检查值是否未定义或为空的详细方法是：

return value === undefined || value === null;

You can also use the ==operator but this expects one to know all the rules:

您也可以使用==运算符，但这需要知道所有规则：

return value == null; // also returns true if value is undefined

回答by Mike Samuel

function isEmpty(value){
  return (value == null || value.length === 0);
}

This will return true for

这将返回 true

undefined  // Because undefined == null

null

[]

""

and zero argument functions since a function's lengthis the number of declared parameters it takes.

和零参数函数，因为函数length是它所采用的声明参数的数量。

To disallow the latter category, you might want to just check for blank strings

要禁止后一类，您可能只想检查空白字符串

function isEmpty(value){
  return (value == null || value === '');
}

回答by guya

This is the safest check and I haven't seen it posted here exactly like that:

这是最安全的检查，我还没有看到它完全像这样发布在这里：

if (typeof value !== 'undefined' && value) {
    //deal with value'
};

It will cover cases where valuewas never defined, and also any of these:

它将涵盖从未定义值的情况，以及以下任何一种情况：

• null
• undefined (value of undefined is not the same as a parameter that was never defined)
• 0
• "" (empty string)
• false
• NaN

• 空值
• 未定义（未定义的值与从未定义的参数不同）
• 0
• ""（空字符串）
• 错误的
• NaN

_{Edited: Changed to strict equality (!==) because it's the norm by now ;)}

_{编辑：更改为严格相等（！==），因为它现在是规范；）}

回答by Vix

You may find the following function useful:

您可能会发现以下功能很有用：

function typeOf(obj) {
  return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}

Or in ES7 (comment if further improvements)

或者在 ES7 中（如果有进一步改进请评论）

function typeOf(obj) {
  const { toString } = Object.prototype;
  const stringified = obj::toString();
  const type = stringified.split(' ')[1].slice(0, -1);

  return type.toLowerCase();
}

Results:

结果：

typeOf(); //undefined
typeOf(null); //null
typeOf(NaN); //number
typeOf(5); //number
typeOf({}); //object
typeOf([]); //array
typeOf(''); //string
typeOf(function () {}); //function
typeOf(/a/) //regexp
typeOf(new Date()) //date
typeOf(new WeakMap()) //weakmap
typeOf(new Map()) //map

"Note that the bind operator (::) is not part of ES2016 (ES7) nor any later edition of the ECMAScript standard at all. It's currently a stage 0 (strawman) proposal for being introduced to the language." – Simon Kjellberg. the author wishes to add his support for this beautiful proposal to receive royal ascension.

“请注意，绑定运算符 (::) 不是 ES2016 (ES7) 的一部分，也不是 ECMAScript 标准的任何更高版本。它目前是引入该语言的第 0 阶段（稻草人）提案。” – 西蒙·凯尔伯格。作者希望支持这项美丽的升职提议。

回答by krupar

The first answer with best rating is wrong. If value is undefined it will throw an exception in modern browsers. You have to use:

评分最高的第一个答案是错误的。如果 value 未定义，它将在现代浏览器中引发异常。你必须使用：

if (typeof(value) !== "undefined" && value)

or

或者

if (typeof value  !== "undefined" && value)

回答by Arif

!check for empty strings (""), null, undefined, false and the number 0 and NaN.Say, if a string is empty var name = ""then console.log(!name)returns true.

！检查空字符串 ("")、null、undefined、false 以及数字 0 和 NaN。比如说，如果一个字符串为空，var name = ""则console.log(!name)返回true.

function isEmpty(val){
  return !val;
}

this function will return true if valis empty, null, undefined, false, the number 0 or NaN.

如果val为空、null、undefined、false、数字 0 或 NaN，则此函数将返回 true 。

OR

或者

According to your problem domain you can just use like !valor !!val.

根据您的问题域，您可以使用 like!val或!!val。

回答by JerryP

This condition check

这个条件检查

if (!!foo) {
    //foo is defined
}

is all you need.

是你所需要的全部。

回答by Alexandre Magro

A solution I like a lot:

我非常喜欢的一个解决方案：

Let's define that a blank variable is null, or undefined, or if it has length, it is zero, or if it is an object, it has no keys:

让我们定义一个空白变量是null, or undefined，或者如果它有长度，它为零，或者如果它是一个对象，它没有键：

function isEmpty (value) {
  return (
    // null or undefined
    (value == null) ||

    // has length and it's zero
    (value.hasOwnProperty('length') && value.length === 0) ||

    // is an Object and has no keys
    (value.constructor === Object && Object.keys(value).length === 0)
  )
}

Returns:

返回：

• true:undefined, null, "", [], {}
• false:true, false, 1, 0, -1, "foo", [1, 2, 3], { foo: 1 }

• 真:undefined, null, "", [],{}
• 假:true, false, 1, 0, -1, "foo", [1, 2, 3],{ foo: 1 }

回答by tcooc

You are a bit overdoing it. To check if a variable is not given a value, you would only need to check against undefined and null.

你有点过分了。要检查变量是否没有赋值，您只需要检查 undefined 和 null。

function isEmpty(value){
    return (typeof value === "undefined" || value === null);
}

This is assuming 0, "", and objects(even empty object and array) are valid "values".

这是假设0,""和 对象（甚至空对象和数组）是有效的“值”。