How can I check for an empty/undefined/null string in JavaScript?

You can check the value of the item if you only want to do so.

if (strValue) {
    //do something
}

If you have to verify that null is not an empty string, then I think you should check against "" , using the === operator.

if (strValue === "") {
    //...
}

I use the following to check if a string has null, empty, or undefined:

function isEmpty(str) {
    return (!str || 0 === str.length);
}

I use this method to check if a string has null, blank, or undefined.

function isBlank(str) {
    return (!str || /^\s*$/.test(str));
}

You can check whether a string has white-space or is blank.

String.prototype.isEmpty = function() {
    return (this.length === 0 || !this.trim());
};

The answers to the above questions are excellent, but this one will be even better. Use dual NOT operators (!! Use dual NOT operators  (!!):

if (!!str) {
    // Some code here
}

Or, you can use type casting

if (Boolean(str)) {
    // Code here
}

Both have the same function. Typecast the variable into Boolean.  str is a variable.

It returns  false for null, undefined, 0, 000,

It holds true  for strings  "0"  and whitespace " ".