[javascript] How to check if a variable is null or empty string or all whitespace in JavaScript?

I need to check to see if a variable is null or has all empty spaces or is just blank ("").

I have the following, but it is not working:

var addr;
addr = "  ";

if (!addr) {
    // pull error 
}

If I do the following, it works:

if (addr) {

}?

What I need is something like the C# method String.IsNullOrWhiteSpace(value).

function isEmptyOrSpaces(str){
    return str === null || str.match(/^[\s\n\r]*$/) !== null;
}

When checking for white space the c# method uses the Unicode standard. White space includes spaces, tabs, carriage returns and many other non-printing character codes. So you are better of using:

function isNullOrWhiteSpace(str){
    return str == null || str.replace(/\s/g, '').length < 1;
}

You can use if(addr && (addr = $.trim(addr)))

This has the advantage of actually removing any outer whitespace from addr instead of just ignoring it when performing the check.

Reference: http://api.jquery.com/jQuery.trim/

if (addr == null || addr.trim() === ''){
  //...
}

A null comparison will also catch undefined. If you want false to pass too, use !addr. For backwards browser compatibility swap addr.trim() for $.trim(addr).

You can try this:

do {
   var op = prompt("please input operatot \n you most select one of * - / *  ")
} while (typeof op == "object" || op == ""); 
// execute block of code when click on cancle or ok whthout input

You can create your own method Equivalent to

String.IsNullOrWhiteSpace(value)

function IsNullOrWhiteSpace( value) {

    if (value== null) return true;

    return value.replace(/\s/g, '').length == 0;
}

isEmptyOrSpaces(str){
    return str === null || str.trim().length>0;
}

Old question, but I think it deservers a simpler answer.

You can simply do:

var addr = "  ";

if (addr && addr.trim()) {

    console.log("I'm not null, nor undefined, nor empty string, nor string composed of whitespace only.");

}

isEmptyOrSpaces(str){
    return !str || str.trim() === '';
}

Simplified version of the above: (from here: https://stackoverflow.com/a/32800728/47226)

function isNullOrWhitespace( input ) {
  return !input || !input.trim();
}

Try this out

/**
  * Checks the string if undefined, null, not typeof string, empty or space(s)
  * @param {any} str string to be evaluated
  * @returns {boolean} the evaluated result
*/
function isStringNullOrWhiteSpace(str) {
    return str === undefined || str === null
                             || typeof str !== 'string'
                             || str.match(/^ *$/) !== null;
}

You can use it like this

isStringNullOrWhiteSpace('Your String');