如何在JavaScript中检查空值?我写了下面的代码,但没用。
if (pass == null || cpass == null || email == null || cemail == null || user == null) {
alert("fill all columns");
return false;
}
如何在JavaScript程序中查找错误?
如何在JavaScript中检查空值?我写了下面的代码,但没用。
if (pass == null || cpass == null || email == null || cemail == null || user == null) {
alert("fill all columns");
return false;
}
如何在JavaScript程序中查找错误?
当前回答
JavaScript在检查“空”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,这个更简单的代码会起作用:
if(!pass || !cpass || !email || !cemail || !user){
它将检查空字符串(“”)、null、undefined、false以及数字0和NaN。
请注意,如果您是专门检查数字,则使用此方法错过0是一个常见错误,num!==对于返回-1的函数(例如indexOf),首选0(或num!==-1或~num(同时检查-1的黑客代码))。
其他回答
“Object.is()”方法可用于确定两个值是否相同。因此,您可以使用它来检查对象是否为空。
检查空值
let testA = null; //null
console.log(Object.is(testA, null)); //true //null === null
if(Object.is(testA, null)) {
console.log("This is a Null Value");
}
Output:
true
This is a Null Value
检查未定义的值
let testB; //undefined
console.log(Object.is(testB, undefined)); //true //undefined === undefined
if(Object.is(testB, undefined)) {
console.log("This is an undefined Value");
}
Output:
true
This is an undefined Value
如果要同时检查undefined和null,请使用此选项。
let testC; //undefined
console.log(Object.is(testC, undefined)); //true //undefined === undefined
console.log(Object.is(testC, null)); //false //undefined === null
if (Object.is(testC, undefined) || Object.is(testC, null)){
console.log("The value is either undefined or null");
}
if (!(Object.is(testC, undefined) || Object.is(testC, null))){
console.log("The value is neither undefined nor null");
}
Output:
true
false
The value is either undefined or null
看看他们:https://onecompiler.com/javascript/3ymdqd34v
Mozilla解释:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is
您可以检查某些值是否为空,如下所示
[pass,cpass,email,cemail,user].some(x=> x===null)
let pass=1;设cpass=2;让电子邮件=3;let cemail=空;让用户=5;如果([pass,cpass,email,cemail,user].some(x=>x==null)){警报(“填充所有列”);//return false;}
奖金:为什么==比==更清楚(来源)
a==b
a===b
我找到了另一种方法来测试该值是否为空:
if(variable >= 0 && typeof variable === "object")
null同时充当数字和对象。比较null>=0或null<=0结果为true。比较null==0或null>0或null<0将导致false。但由于null也是一个对象,我们可以将其检测为null。
我做了一个更复杂的函数性质,它比typeof做得更好,并且可以被告知要包含或保持分组的类型
/*函数性质of(变量,[包含类型])包括的类型有null-null将导致“未定义”,如果包含,则将导致“null”NaN-NaN将导致“未定义”,或如果包含,将导致“NaN”-infinity-将负-无穷与“无限”分开number-将数字拆分为“int”或“double”array-将“array”与“object”分开空-空的“字符串”将导致“空”或空=未定义-空“字符串”将导致“未定义”*/函数性质of(v,…类型){/*null*/if(v===null)返回类型。includes('null')?“null”:“未定义”;/*NaN*/if(typeof v==“number”)返回(isNaN(v))?类型。包括('NaN')?“NaN”:“未定义”:/*-无穷大*/(v+1===v)?(types.includes('-infinity')&&v==Number.NEGATIVE_infinity)?“-无限”:“无限”:/*number*/(types.includes('number'))?(数字.isInteger(v))?“int”:“double”:“number”;/*array*/if(typeof v==“object”)return(types.includes('array')&&array.isArray(v))?“array”:“object”;/*空*/if(typeof v==“string”)返回(v==”)?types.includes(“空”)?“空”:/*空=未定义*/types.includes('empty=未定义')?“undefined”:“string”:“字符串”;否则返回v的类型}//演示let types=[null,“”,“string”,undefined,NaN,Infinity,-Invinity,false,“false”,true,“true”,0,1,-1,0.1,“test”,{var:1},[1,2],{0:1,1:2,长度:2}]for(i在类型中){console.log(“natureof”,类型[i],“=”,natureof(类型[i]”,“null”,“NaN”,“-ninfinity”,“number”,“array”,“empty=undefined”))}
检查错误条件:
// Typical API response data
let data = {
status: true,
user: [],
total: 0,
activity: {sports: 1}
}
// A flag that checks whether all conditions were met or not
var passed = true;
// Boolean check
if (data['status'] === undefined || data['status'] == false){
console.log("Undefined / no `status` data");
passed = false;
}
// Array/dict check
if (data['user'] === undefined || !data['user'].length){
console.log("Undefined / no `user` data");
passed = false;
}
// Checking a key in a dictionary
if (data['activity'] === undefined || data['activity']['time'] === undefined){
console.log("Undefined / no `time` data");
passed = false;
}
// Other values check
if (data['total'] === undefined || !data['total']){
console.log("Undefined / no `total` data");
passed = false;
}
// Passed all tests?
if (passed){
console.log("Passed all tests");
}
要检查空值,请使用以下命令:
if (variable === null)
此测试仅通过null,不通过“”、undefined、false、0或NaN。
此外,我还为每个“类假”值提供了绝对检查(对于!变量返回true)。
注意,对于某些绝对检查,您需要使用绝对等于:==和typeof。
我在这里创建了一个JSFiddle,以显示所有单独测试的工作情况
以下是每次检查的输出:
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
正如你所看到的,测试NaN有点困难;