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Accepted
Rubén
Asked: 2020-02-13 16:44:43 +0800 CST 2020-02-13 16:44:43 +0800 CST

How to determine if a variable is 'undefined' or 'null' in JavaScript

  • 772

I want to determine if a variable is undefined or has the value null.

In the following example, I want the clause corresponding to the true condition to be executed, that is, the message "EmpName is not defined" is printed on the console, however, the clause in "else" is already executed. which prints "EmpName is defined"

var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
  console.log("EmpName no está definido");
} else {
  console.log("EmpName está definido");
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<div id="esd-names">
  <div id="name"></div>
</div>

var miVariable; //miVariable está definida pero tiene valor nulo.
console.log(miVariable)

What is the right way?

javascript

6 Answers

  1. The value undefinedis not the same as the string 'undefined'. In your example, I would do:

    var EmpName = $("div#esd-names div#name").attr('class');
if(typeof EmpName === 'undefined'){
  console.log("EmpName no está definido");
} else {
  console.log("EmpName está definido");
}
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<div id="esd-names">
  <div id="name"></div>
</div>

    O well

    var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName === undefined){
  console.log("EmpName no está definido");
} else {
  console.log("EmpName está definido");
}
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<div id="esd-names">
  <div id="name"></div>
</div>

    • 9
  2. Best Answer

    Short answer

    Instead of 'undefined'(string) use null(global object).

    Explanation

    'undefined'and nullthey are not the same nor are they of the same type. The first refers to a string and the second is a global object (see null , Mozilla Developers Network article).

    NOTE: See amandiel 's answer which clarifies the difference between (string) and (reserved word) or undefined (Mozilla Developers Network article).'undefined'undefined

    Fragment

    In the following example it has been replaced 'undefined'by nulland the messages to be printed have been adjusted for each case.

    var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == null){
  console.log("El valor de EmpName " + EmpName); //Nulo
} else {
  console.log("El valor de EmpName es " + EmpName); //Cualquier otro
}
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<div id="esd-names">
  <div id="name"></div>
</div>

    Another more general way is to replace the comparison ( EmpName == 'undefined') with the name of the variable ( EmpName)

    var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName){
  console.log("El valor de EmpName es " + EmpName); //Cualquier valor
} else {
  console.log("El valor de EmpName es " + EmpName); //No definido
}
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<div id="esd-names">
  <div id="name"></div>
</div>

    • 4

  3. Since javascriptthere is no equivalent to issetas in , phpyou must take into account the following:

    If the value of the variable is undefineduse:

    if(variable === undefined){
...
}

    If the value of the variable is nulluse:

    if(variable === null){
...
}

    If the value of the variable is empty:

    if(variable === ""){
...
}

    You can also use:

    if(variable.isEmpty()){
...
}

    If you don't know what type of value the variable has, you can get it with:

    console.log(variable)

    • 4

  4. You can try it like this:

    var EmpName = $("div#esd-names div#name").attr('class');
if (EmpName == null || EmpName == undefined){
  console.log("EmpName es NULL o no está definido");
} else {
  console.log("EmpName está definido");
}
    • 1 
  5. var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == null){
    console.log("EmpName no está definido");
} else {
    console.log("EmpName está definido");
}
    • 0 
  6. var dato=$("#dato").val('');
  if(dato==''){
     alert("no esta definido");
  }else{
     alert("esta definido");
  }
    • -1