如何检查 JavaScript 中的空值？

javascript equality comparison null compare

2022-08-29 22:27:20

如何在 JavaScript 中检查空值？我写了下面的代码，但它不起作用。

if (pass == null || cpass == null || email == null || cemail == null || user == null) {      

    alert("fill all columns");
    return false;  

}

如何在我的JavaScript程序中找到错误？

答案 1

JavaScript在检查“null”值方面非常灵活。我猜你实际上是在寻找空字符串，在这种情况下，这个更简单的代码将起作用：

if(!pass || !cpass || !email || !cemail || !user){

这将检查空字符串（）、、、和数字和。""nullundefinedfalse0NaN

请注意，如果您专门检查数字，则使用此方法会错过这是一个常见的错误，并且对于返回的函数（例如）是首选（或或（也检查）的黑客代码）0num !== 0num !== -1~num-1-1indexOf

答案 2

要专门检查空值，您将使用以下命令：

if (variable === null)

此测试将仅通过，、、或。null""undefinedfalse0NaN

此外，我还为每个“类似 false”的值提供了绝对检查（一个将返回 true 的值）。!variable

请注意，对于某些绝对检查，您需要实现使用和。absolutely equals: ===typeof

我在这里创建了一个JSFiddle来展示所有单独的测试工作

以下是每个检查的输出：

Null Test:

if (variable === null)

- variable = ""; (false) typeof variable = string

- variable = null; (true) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



Empty String Test:

if (variable === '')

- variable = ''; (true) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number




Undefined Test:

if (typeof variable == "undefined")

-- or --

if (variable === undefined)

- variable = ''; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (true) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



False Test:

if (variable === false)

- variable = ''; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (true) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



Zero Test:

if (variable === 0)

- variable = ''; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (true) typeof variable = number

- variable = NaN; (false) typeof variable = number



NaN Test:

if (typeof variable == 'number' && !parseFloat(variable) && variable !== 0)

-- or --

if (isNaN(variable))

- variable = ''; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (true) typeof variable = number

如您所见，测试起来有点困难NaN;