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

Empty string, undefined, null, ...

To check for a truthy value:

if (strValue) {
    // strValue was non-empty string, true, 42, Infinity, [], ...
}

To check for a falsy value:

if (!strValue) {
    // strValue was empty string, false, 0, null, undefined, ...
}

Empty string (only!)

To check for exactly an empty string, compare for strict equality against "" using the === operator:

if (strValue === "") {
    // strValue was empty string
}

To check for not an empty string strictly, use the !== operator:

if (strValue !== "") {
    // strValue was not an empty string
}

For checking if a variable is falsey or if it has length attribute equal to zero (which for a string, means it is empty), I use:

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

_{(Note that strings aren't the only variables with a length attribute, arrays have them as well, for example.)}

Alternativaly, you can use the (not so) newly optional chaining and arrow functions to simplify:

const isEmpty = (str) => (!str?.length);

It will check the length, returning undefined in case of a nullish value, without throwing an error. In the case of an empty value, zero is falsy and the result is still valid.

For checking if a variable is falsey or if the string only contains whitespace or is empty, I use:

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

If you want, you can monkey-patch the String prototype like this:

String.prototype.isEmpty = function() {
    // This doesn't work the same way as the isEmpty function used 
    // in the first example, it will return true for strings containing only whitespace
    return (this.length === 0 || !this.trim());
};
console.log("example".isEmpty());

_{Note that monkey-patching built-in types are controversial, as it can break code that depends on the existing structure of built-in types, for whatever reason.}

All the previous answers are good, but this will be even better. Use dual NOT operators (!!):

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

Or use type casting:

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

Both do the same function. Typecast the variable to Boolean, where str is a variable.

• It returns false for null, undefined, 0, 000, "", false.

• It returns true for all string values other than the empty string (including strings like "0" and " ")

The closest thing you can get to str.Empty (with the precondition that str is a String) is:

if (!str.length) { ...

If you need to make sure that the string is not just a bunch of empty spaces (I'm assuming this is for form validation) you need to do a replace on the spaces.

if(str.replace(/\s/g,"") == ""){
}

I use:

function empty(e) {
  switch (e) {
    case "":
    case 0:
    case "0":
    case null:
    case false:
    case undefined:
      return true;
    default:
      return false;
  }
}

empty(null) // true
empty(0) // true
empty(7) // false
empty("") // true
empty((function() {
    return ""
})) // false

Performance

I perform tests on macOS v10.13.6 (High Sierra) for 18 chosen solutions. Solutions works slightly different (for corner-case input data) which was presented in the snippet below.

Conclusions

• the simple solutions based on !str,==,=== and length are fast for all browsers (A,B,C,G,I,J)
• the solutions based on the regular expression (test,replace) and charAt are slowest for all browsers (H,L,M,P)
• the solutions marked as fastest was fastest only for one test run - but in many runs it changes inside 'fast' solutions group

Details

In the below snippet I compare results of chosen 18 methods by use different input parameters

• "" "a" " "- empty string, string with letter and string with space
• [] {} f- array, object and function
• 0 1 NaN Infinity - numbers
• true false - Boolean
• null undefined

Not all tested methods support all input cases.

function A(str) {
  let r=1;
  if (!str)
    r=0;
  return r;
}

function B(str) {
  let r=1;
  if (str == "")
    r=0;
  return r;
}

function C(str) {
  let r=1;
  if (str === "")
    r=0;
  return r;
}

function D(str) {
  let r=1;
  if(!str || 0 === str.length)
    r=0;
  return r;
}

function E(str) {
  let r=1;
  if(!str || /^\s*$/.test(str))
    r=0;
  return r;
}

function F(str) {
  let r=1;
  if(!Boolean(str))
    r=0;
  return r;
}

function G(str) {
  let r=1;
  if(! ((typeof str != 'undefined') && str) )
    r=0;
  return r;
}

function H(str) {
  let r=1;
  if(!/\S/.test(str))
    r=0;
  return r;
}

function I(str) {
  let r=1;
  if (!str.length)
    r=0;
  return r;
}

function J(str) {
  let r=1;
  if(str.length <= 0)
    r=0;
  return r;
}

function K(str) {
  let r=1;
  if(str.length === 0 || !str.trim())
    r=0;
  return r;
}

function L(str) {
  let r=1;
  if ( str.replace(/\s/g,"") == "")
    r=0;
  return r;
}

function M(str) {
  let r=1;
  if((/^\s*$/).test(str))
    r=0;
  return r;
}


function N(str) {
  let r=1;
  if(!str || !str.trim().length)
    r=0;
  return r;
}

function O(str) {
  let r=1;
  if(!str || !str.trim())
    r=0;
  return r;
}

function P(str) {
  let r=1;
  if(!str.charAt(0))
    r=0;
  return r;
}

function Q(str) {
  let r=1;
  if(!str || (str.trim()==''))
    r=0;
  return r;
}

function R(str) {
  let r=1;
  if (typeof str == 'undefined' ||
      !str ||
      str.length === 0 ||
      str === "" ||
      !/[^\s]/.test(str) ||
      /^\s*$/.test(str) ||
      str.replace(/\s/g,"") === "")

    r=0;
  return r;
}




// --- TEST ---

console.log(                  '   ""  "a"  " " [] {} 0 1 NaN Infinity f true false null undefined ');
let log1 = (s,f)=> console.log(`${s}: ${f("")}   ${f("a")}    ${f(" ")}   ${f([])}  ${f({})}  ${f(0)} ${f(1)} ${f(NaN)}   ${f(Infinity)}        ${f(f)} ${f(true)}    ${f(false)}     ${f(null)}    ${f(undefined)}`);
let log2 = (s,f)=> console.log(`${s}: ${f("")}   ${f("a")}    ${f(" ")}   ${f([])}  ${f({})}  ${f(0)} ${f(1)} ${f(NaN)}   ${f(Infinity)}        ${f(f)} ${f(true)}    ${f(false)}`);
let log3 = (s,f)=> console.log(`${s}: ${f("")}   ${f("a")}    ${f(" ")}`);

log1('A', A);
log1('B', B);
log1('C', C);
log1('D', D);
log1('E', E);
log1('F', F);
log1('G', G);
log1('H', H);

log2('I', I);
log2('J', J);

log3('K', K);
log3('L', L);
log3('M', M);
log3('N', N);
log3('O', O);
log3('P', P);
log3('Q', Q);
log3('R', R);

And then for all methods I perform speed test case str = "" for browsers Chrome v78.0.0, Safari v13.0.4, and Firefox v71.0.0 - you can run tests on your machine here

You can use lodash: _.isEmpty(value).

It covers a lot of cases like {}, '', null, undefined, etc.

But it always returns true for Number type of JavaScript primitive data types like _.isEmpty(10) or _.isEmpty(Number.MAX_VALUE) both returns true.

Very generic "All-In-One" Function (not recommended though):

function is_empty(x)
{
    return (                                                           //don't put newline after return
        (typeof x == 'undefined')
              ||
        (x == null)
              ||
        (x == false)        //same as: !x
              ||
        (x.length == 0)
              ||
        (x == 0)            // note this line, you might not need this. 
              ||
        (x == "")
              ||
        (x.replace(/\s/g,"") == "")
              ||
        (!/[^\s]/.test(x))
              ||
        (/^\s*$/.test(x))
    );
}

However, I don't recommend to use that, because your target variable should be of specific type (i.e. string, or numeric, or object?), so apply the checks that are relative to that variable.

var s; // undefined
var s = ""; // ""
s.length // 0

There's nothing representing an empty string in JavaScript. Do a check against either length (if you know that the var will always be a string) or against ""

Try:

if (str && str.trim().length) {  
    //...
}

I would not worry too much about the most efficient method. Use what is most clear to your intention. For me that's usually strVar == "".

As per the comment from Constantin, if strVar could some how end up containing an integer 0 value, then that would indeed be one of those intention-clarifying situations.

A lot of answers, and a lot of different possibilities!

Without a doubt for quick and simple implementation the winner is: if (!str.length) {...}

However, as many other examples are available. The best functional method to go about this, I would suggest:

function empty(str)
{
    if (typeof str == 'undefined' || !str || str.length === 0 || str === "" || !/[^\s]/.test(str) || /^\s*$/.test(str) || str.replace(/\s/g,"") === "")
        return true;
    else
        return false;
}

A bit excessive, I know.

1. check that var a; exist

2. trim out the false spaces in the value, then test for emptiness

   if ((a)&&(a.trim()!=''))
   {
     // if variable a is not empty do this 
   }
   

You could also go with regular expressions:

if((/^\s*$/).test(str)) { }

Checks for strings that are either empty or filled with whitespace.

There is a lot of useful information here, but in my opinion, one of the most important elements was not addressed.

null, undefined, and "" are all falsy.

When evaluating for an empty string, it's often because you need to replace it with something else.

In which case, you can expect the following behavior.

var a = ""
var b = null
var c = undefined

console.log(a || "falsy string provided") // prints ->"falsy string provided"
console.log(b || "falsy string provided") // prints ->"falsy string provided"
console.log(c || "falsy string provided") // prints ->"falsy string provided"

With that in mind, a method or function that can return whether or not a string is "", null, or undefined (an invalid string) versus a valid string is as simple as this:

const validStr = (str) => str ? true : false

validStr(undefined) // returns false
validStr(null) // returns false
validStr("") // returns false
validStr("My String") // returns true

if ((input?.trim()?.length || 0) > 0) {
   // input must not be any of:
   // undefined
   // null
   // ""
   // " " or just whitespace
}

Or in function form:

const isNotNilOrWhitespace = input => (input?.trim()?.length || 0) > 0;

const isNilOrWhitespace = input => (input?.trim()?.length || 0) === 0;

Explanation:

If input is undefined or null then the null coalescing ?. will result in input?.trim()?.length will be undefined or null. ORing (||) that with 0 will give 0. 0 is not > 0 therefore the result will be false, ie it IS a nil value.

If input is empty or whitespace then .trim() will remove leading and ending whitespace, which will keep an empty input the same, and convert any whitespace to an empty value. The length of an empty string is then 0, and as above, 0 is not > 0, therefore the result will be false, ie it IS empty or only whitespace.

If input is any other string, it's length will be > 0 after calling .trim(), and therefore the result will be true, ie it IS NOT a nil value, and it IS NOT empty or only whitespace.

I usually use something like this,

if (!str.length) {
    // Do something
}

Also, in case you consider a whitespace filled string as "empty".

You can test it with this regular expression:

!/\S/.test(string); // Returns true if blank.

If one needs to detect not only empty but also blank strings, I'll add to Goral's answer:

function isEmpty(s){
    return !s.length;    
}

function isBlank(s){
    return isEmpty(s.trim());    
}

Starting with:

return (!value || value == undefined || value == "" || value.length == 0);

Looking at the last condition, if value == "", its length must be 0. Therefore drop it:

return (!value || value == undefined || value == "");

But wait! In JavaScript, an empty string is false. Therefore, drop value == "":

return (!value || value == undefined);

And !undefined is true, so that check isn't needed. So we have:

return (!value);

And we don't need parentheses:

return !value

I use a combination, and the fastest checks are first.

function isBlank(pString) {
    if (!pString) {
        return true;
    }
    // Checks for a non-white space character
    // which I think [citation needed] is faster
    // than removing all the whitespace and checking
    // against an empty string
    return !/[^\s]+/.test(pString);
}

I have not noticed an answer that takes into account the possibility of null characters in a string. For example, if we have a null character string:

var y = "\0"; // an empty string, but has a null character
(y === "") // false, testing against an empty string does not work
(y.length === 0) // false
(y) // true, this is also not expected
(y.match(/^[\s]*$/)) // false, again not wanted

To test its nullness one could do something like this:

String.prototype.isNull = function(){ 
  return Boolean(this.match(/^[\0]*$/)); 
}
...
"\0".isNull() // true

It works on a null string, and on an empty string and it is accessible for all strings. In addition, it could be expanded to contain other JavaScript empty or whitespace characters (i.e. nonbreaking space, byte order mark, line/paragraph separator, etc.).

Meanwhile we can have one function that checks for all 'empties' like null, undefined, '', ' ', {}, []. So I just wrote this.

var isEmpty = function(data) {
    if(typeof(data) === 'object'){
        if(JSON.stringify(data) === '{}' || JSON.stringify(data) === '[]'){
            return true;
        }else if(!data){
            return true;
        }
        return false;
    }else if(typeof(data) === 'string'){
        if(!data.trim()){
            return true;
        }
        return false;
    }else if(typeof(data) === 'undefined'){
        return true;
    }else{
        return false;
    }
}

Use cases and results.

console.log(isEmpty()); // true
console.log(isEmpty(null)); // true
console.log(isEmpty('')); // true
console.log(isEmpty('  ')); // true
console.log(isEmpty(undefined)); // true
console.log(isEmpty({})); // true
console.log(isEmpty([])); // true
console.log(isEmpty(0)); // false
console.log(isEmpty('Hey')); // false

I did some research on what happens if you pass a non-string and non-empty/null value to a tester function. As many know, (0 == "") is true in JavaScript, but since 0 is a value and not empty or null, you may want to test for it.

The following two functions return true only for undefined, null, empty/whitespace values and false for everything else, such as numbers, Boolean, objects, expressions, etc.

function IsNullOrEmpty(value)
{
    return (value == null || value === "");
}
function IsNullOrWhiteSpace(value)
{
    return (value == null || !/\S/.test(value));
}

More complicated examples exists, but these are simple and give consistent results. There is no need to test for undefined, since it's included in (value == null) check. You may also mimic C# behaviour by adding them to String like this:

String.IsNullOrEmpty = function (value) { ... }

You do not want to put it in Strings prototype, because if the instance of the String-class is null, it will error:

String.prototype.IsNullOrEmpty = function (value) { ... }
var myvar = null;
if (1 == 2) { myvar = "OK"; } // Could be set
myvar.IsNullOrEmpty(); // Throws error

I tested with the following value array. You can loop it through to test your functions if in doubt.

// Helper items
var MyClass = function (b) { this.a = "Hello World!"; this.b = b; };
MyClass.prototype.hello = function () { if (this.b == null) { alert(this.a); } else { alert(this.b); } };
var z;
var arr = [
// 0: Explanation for printing, 1: actual value
    ['undefined', undefined],
    ['(var) z', z],
    ['null', null],
    ['empty', ''],
    ['space', ' '],
    ['tab', '\t'],
    ['newline', '\n'],
    ['carriage return', '\r'],
    ['"\\r\\n"', '\r\n'],
    ['"\\n\\r"', '\n\r'],
    ['" \\t \\n "', ' \t \n '],
    ['" txt \\t test \\n"', ' txt \t test \n'],
    ['"txt"', "txt"],
    ['"undefined"', 'undefined'],
    ['"null"', 'null'],
    ['"0"', '0'],
    ['"1"', '1'],
    ['"1.5"', '1.5'],
    ['"1,5"', '1,5'], // Valid number in some locales, not in JavaScript
    ['comma', ','],
    ['dot', '.'],
    ['".5"', '.5'],
    ['0', 0],
    ['0.0', 0.0],
    ['1', 1],
    ['1.5', 1.5],
    ['NaN', NaN],
    ['/\S/', /\S/],
    ['true', true],
    ['false', false],
    ['function, returns true', function () { return true; } ],
    ['function, returns false', function () { return false; } ],
    ['function, returns null', function () { return null; } ],
    ['function, returns string', function () { return "test"; } ],
    ['function, returns undefined', function () { } ],
    ['MyClass', MyClass],
    ['new MyClass', new MyClass()],
    ['empty object', {}],
    ['non-empty object', { a: "a", match: "bogus", test: "bogus"}],
    ['object with toString: string', { a: "a", match: "bogus", test: "bogus", toString: function () { return "test"; } }],
    ['object with toString: null', { a: "a", match: "bogus", test: "bogus", toString: function () { return null; } }]
];

I didn't see a good answer here (at least not an answer that fits for me)

So I decided to answer myself:

value === undefined || value === null || value === "";

You need to start checking if it's undefined. Otherwise your method can explode, and then you can check if it equals null or is equal to an empty string.

You cannot have !! or only if(value) since if you check 0 it's going to give you a false answer (0 is false).

With that said, wrap it up in a method like:

public static isEmpty(value: any): boolean { return value === undefined || value === null || value === ""; }

PS.: You don't need to check typeof, since it would explode and throw even before it enters the method

Try this:

export const isEmpty = string => (!string || !string.length);

All these answers are nice.

But I cannot be sure that variable is a string, doesn't contain only spaces (this is important for me), and can contain '0' (string).

My version:

function empty(str){
    return !str || !/[^\s]+/.test(str);
}

empty(null); // true
empty(0); // true
empty(7); // false
empty(""); // true
empty("0"); // false
empty("  "); // true

Sample on jsfiddle.

Trimming whitespace with the null-coalescing operator:

if (!str?.trim()) {
  // do something...
}

There's no isEmpty() method, you have to check for the type and the length:

if (typeof test === 'string' && test.length === 0){
  ...

The type check is needed in order to avoid runtime errors when test is undefined or null.