How Can I Effectively Check for Undefined or Null Variables in JavaScript?

Determining Whether a Variable is 'Undefined' or 'Null'

When attempting to determine if a variable is undefined or null, the abstract equality operator can be employed. The abstract equality operator (==) compares two values by performing type coercion and evaluates to true if the resulting values are the same. This property can be leveraged to distinguish between undefined and null.

Consider the following code snippet:

var EmpName = $("#esd-names div#name").attr('class');
if (EmpName == 'undefined') {
  // DO SOMETHING
};

In this code, the attempt is made to compare the value of EmpName to the string 'undefined'. However, this results in a JavaScript interpreter error because the value of EmpName is undefined, not a string.

To resolve this issue, the abstract equality operator can be used:

if (EmpName == null) {
  // your code here.
}

Since null == undefined evaluates to true, this code will successfully identify both null and undefined values for EmpName.

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