This is a bigger question than you think. Variables can empty in a lot of ways. Kinda depends on what you need to know.

// quick and dirty will be true for '', null, undefined, 0, NaN and false.
if (!x) 

// test for null OR undefined
if (x == null)  

// test for undefined OR null 
if (x == undefined) 

// test for undefined
if (x === undefined) 
// or safer test for undefined since the variable undefined can be set causing tests against it to fail.
if (typeof x == 'undefined') 

// test for empty string
if (x === '') 

// if you know its an array
if (x.length == 0)  
// or
if (!x.length)

// BONUS test for empty object
var empty = true, fld;
for (fld in x) {
  empty = false;
  break;
}

This should cover all cases:

function empty( val ) {

    // test results
    //---------------
    // []        true, empty array
    // {}        true, empty object
    // null      true
    // undefined true
    // ""        true, empty string
    // ''        true, empty string
    // 0         false, number
    // true      false, boolean
    // false     false, boolean
    // Date      false
    // function  false

        if (val === undefined)
        return true;

    if (typeof (val) == 'function' || typeof (val) == 'number' || typeof (val) == 'boolean' || Object.prototype.toString.call(val) === '[object Date]')
        return false;

    if (val == null || val.length === 0)        // null or 0 length array
        return true;

    if (typeof (val) == "object") {
        // empty object

        var r = true;

        for (var f in val)
            r = false;

        return r;
    }

    return false;
}

I see potential shortcomings in many solutions posted above, so I decided to compile my own.
Note: it uses Array.prototype.some, check your browser support.

Solution below considers variable empty if one of the following is true:

1. JS thinks that variable is equal to false, which already covers many things like 0, "", [], and even [""] and [0]
2. Value is null or it's type is 'undefined'
3. It is an empty Object

4. It is an Object/Array consisting only of values that are empty themselves (i.e. broken down to primitives each part of it equals false). Checks drill recursively into Object/Array structure. E.g.

   isEmpty({"": 0}) // true
   isEmpty({"": 1}) // false
   isEmpty([{}, {}])  // true
   isEmpty(["", 0, {0: false}]) //true
   

Function code:

/**
 * Checks if value is empty. Deep-checks arrays and objects
 * Note: isEmpty([]) == true, isEmpty({}) == true, isEmpty([{0:false},"",0]) == true, isEmpty({0:1}) == false
 * @param value
 * @returns {boolean}
 */
function isEmpty(value){
  var isEmptyObject = function(a) {
    if (typeof a.length === 'undefined') { // it's an Object, not an Array
      var hasNonempty = Object.keys(a).some(function nonEmpty(element){
        return !isEmpty(a[element]);
      });
      return hasNonempty ? false : isEmptyObject(Object.keys(a));
    }

    return !a.some(function nonEmpty(element) { // check if array is really not empty as JS thinks
      return !isEmpty(element); // at least one element should be non-empty
    });
  };
  return (
    value == false
    || typeof value === 'undefined'
    || value == null
    || (typeof value === 'object' && isEmptyObject(value))
  );
}

Here my simplest solution.

Inspired by PHP empty function

function empty(n){
	return !(!!n ? typeof n === 'object' ? Array.isArray(n) ? !!n.length : !!Object.keys(n).length : true : false);
}

//with number
console.log(empty(0));        //true
console.log(empty(10));       //false

//with object
console.log(empty({}));       //true
console.log(empty({a:'a'}));  //false

//with array
console.log(empty([]));       //true
console.log(empty([1,2]));    //false

//with string
console.log(empty(''));       //true
console.log(empty('a'));      //false

A more readable version of @SJ00 answer:

/**
 * Checks if a JavaScript value is empty
 * @example
 *    isEmpty(null); // true
 *    isEmpty(undefined); // true
 *    isEmpty(''); // true
 *    isEmpty([]); // true
 *    isEmpty({}); // true
 * @param {any} value - item to test
 * @returns {boolean} true if empty, otherwise false
 */
function isEmpty(value) {
    return (
        value === null || // check for null
        value === undefined || // check for undefined
        value === '' || // check for empty string
        (Array.isArray(value) && value.length === 0) || // check for empty array
        (typeof value === 'object' && Object.keys(value).length === 0) // check for empty object
    );
}

See

isEmpty_.isEmpty(object) Returns true if an enumerable object contains no values (no enumerable own-properties). For strings and array-like objects _.isEmpty checks if the length property is 0.

Combining answers from @inkednm into one function:

   function isEmpty(property) {
      return (property === null || property === "" || typeof property === "undefined");
   }

Empty check on a JSON's key depends on use-case. For a common use-case, we can test for following:

1. Not null
2. Not undefined
3. Not an empty String ''
4. Not an empty Object {} [] (Array is an Object)

Function:

function isEmpty(arg){
  return (
    arg == null || // Check for null or undefined
    arg.length === 0 || // Check for empty String (Bonus check for empty Array)
    (typeof arg === 'object' && Object.keys(arg).length === 0) // Check for empty Object or Array
  );
}

Return true for:

isEmpty(''); // Empty String
isEmpty(null); // null
isEmpty(); // undefined
isEmpty({}); // Empty Object
isEmpty([]); // Empty Array

just put the variable inside the if condition, if variable has any value it will return true else false.

if (response.photo){ // if you are checking for string use this if(response.photo == "") condition
 alert("Has Value");
}
else
{
 alert("No Value");
};

What about doing like this.

JSON.stringify({}) === "{}"

It depends on what you mean by "empty". The most common pattern is to check to see if the variable is undefined. Many people also do a null check, for example:
if (myVariable === undefined || myVariable === null)...

or, in a shorter form:
if (myVariable || myVariable === null)...

if (myVar == undefined)

will work to see if the var is declared but not initalized.

Check for undefined:

if (typeof response.photo == "undefined")
{
    // do something
}

This would do the equivelant of vb's IsEmpty. If myvar contains any value, even null, empty string, or 0, it is not "empty".

To check if a variable or property exists, eg it's been declared, though it may be not have been defined, you can use the in operator.

if ("photo" in response)
{
    // do something
}

If you're looking for the equivalent of PHP's empty function, check this out:

function empty(mixed_var) {
  //   example 1: empty(null);
  //   returns 1: true
  //   example 2: empty(undefined);
  //   returns 2: true
  //   example 3: empty([]);
  //   returns 3: true
  //   example 4: empty({});
  //   returns 4: true
  //   example 5: empty({'aFunc' : function () { alert('humpty'); } });
  //   returns 5: false

  var undef, key, i, len;
  var emptyValues = [undef, null, false, 0, '', '0'];

  for (i = 0, len = emptyValues.length; i < len; i++) {
    if (mixed_var === emptyValues[i]) {
      return true;
    }
  }

  if (typeof mixed_var === 'object') {
    for (key in mixed_var) {
      // TODO: should we check for own properties only?
      //if (mixed_var.hasOwnProperty(key)) {
      return false;
      //}
    }
    return true;
  }

  return false;
}

what am I missing if empty array... keyless object... falseness const isEmpty = o => Array.isArray(o) && !o.join('').length || typeof o === 'object' && !Object.keys(o).length || !(+value);

Here's a simpler(short) solution to check for empty variables. This function checks if a variable is empty. The variable provided may contain mixed values (null, undefined, array, object, string, integer, function).

function empty(mixed_var) {
 if (!mixed_var || mixed_var == '0') {
  return true;
 }
 if (typeof mixed_var == 'object') {
  for (var k in mixed_var) {
   return false;
  }
  return true;
 }
 return false;
}

//   example 1: empty(null);
//   returns 1: true

//   example 2: empty(undefined);
//   returns 2: true

//   example 3: empty([]);
//   returns 3: true

//   example 4: empty({});
//   returns 4: true

//   example 5: empty(0);
//   returns 5: true

//   example 6: empty('0');
//   returns 6: true

//   example 7: empty(function(){});
//   returns 7: false

const isEmpty = val => val == null || !(Object.keys(val) || val).length;

My solution:

function isEmpty(object) {
  return (
    (!object)
    || (object === undefined)
    || (object === null)
    || (object === '')
    || ((object?.length !== undefined) && (object.length === 0))
    || (typeof object === 'object' && Object.keys(object).length === 0) 
  );
}

Tests with Jest:

describe('isEmpty should return `false` when the parameter have some truthy value.', () => {
  test('Empty objects should return true', () => {
    expect(utils.isEmpty([])).toBe(true);
    expect(utils.isEmpty({})).toBe(true);
    expect(utils.isEmpty('')).toBe(true);
    expect(utils.isEmpty(undefined)).toBe(true);
    expect(utils.isEmpty(null)).toBe(true);
  });

  test('Truthy objects should return false', () => {
    expect(utils.isEmpty([1])).toBe(false);
    expect(utils.isEmpty({a: undefined})).toBe(false);
    expect(utils.isEmpty({a: 5})).toBe(false);
    expect(utils.isEmpty({a: 5, b: 6, c: undefined})).toBe(false);
    expect(utils.isEmpty('f00')).toBe(false);
    expect(utils.isEmpty('0')).toBe(false);
  });
})

var message_1 = message.trim();
if (message_1.length > 0) { // to do}