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.

This question has two interpretations:

Check if the variable has a value
Check if the variable has a truthy value

The following answers both.

In JavaScript, a value could be nullish or not nullish, and a value could be falsy or truthy.
Nullish values are a proper subset of falsy values:

 ╭─ nullish ──────╮ ╭─ not nullish ─────────────────────────────────╮
┌───────────┬──────┬───────┬───┬────┬─────┬──────┬───┬─────────┬─────┐
│ undefined │ null │ false │ 0 │ "" │ ... │ true │ 1 │ "hello" │ ... │
└───────────┴──────┴───────┴───┴────┴─────┴──────┴───┴─────────┴─────┘
 ╰─ falsy ───────────────────────────────╯ ╰─ truthy ───────────────╯

Check if value is nullish (undefined or null)

Use one of the following depending on your coding style:

if (value == null)                         { /* value is nullish */ }
if (value === undefined || value === null) { /* value is nullish */ }
if (value == undefined)                    { /* value is nullish */ }
if ((value ?? null) === null)              { /* value is nullish */ }

Notes:

• The == operator works because it has a special case for null vs undefined comparison
• The === operator is more readable (opinion based), eqeqeq friendly and allows checking for undefined and null separately
• The first and third examples work identically, however the third one is rarely seen in production code
• The fourth example uses nullish coalescing operator to change nullish values to null for straight forward comparison

Check if value is not nullish

if (value != null)                         { /* value is not nullish, although it could be falsy */ }
if (value !== undefined && value !== null) { /* value is not nullish, although it could be falsy */ }
if (value != undefined)                    { /* value is not nullish, although it could be falsy */ }
if ((value ?? null) !== null)              { /* value is not nullish, although it could be falsy */ }

Check if value is falsy

Use the ! operator:

if (!value) { /* value is falsy */ }

Check if value is truthy

if (value) { /* value is truthy */ }

Data validation

The nullish, falsy and truthy checks cannot be used for data validation on their own. For example, 0 (falsy) is valid age of a person and -1 (truthy) is not. Additional logic needs to be added on case-by-case basis. Some examples:

/*
 * check if value is greater than/equal to 0
 * note that we cannot use truthy check here because 0 must be allowed
 */
[null, -1, 0, 1].forEach(num => {
  if (num != null && num >= 0) {
    console.log("%o is not nullish and greater than/equal to 0", num);
  } else {
    console.log("%o is bad", num);
  }
});

/*
 * check if value is not empty-or-whitespace string
 */
[null, "", " ", "hello"].forEach(str => {
  if (str && /\S/.test(str)) {
    console.log("%o is truthy and has non-whitespace characters", str);
  } else {
    console.log("%o is bad", str);
  }
});

/*
 * check if value is not an empty array
 * check for truthy before checking the length property
 */
[null, [], [1]].forEach(arr => {
  if (arr && arr.length) {
    console.log("%o is truthy and has one or more items", arr);
  } else {
    console.log("%o is bad", arr);
  }
});

/*
 * check if value is not an empty array
 * using optional chaining operator to make sure that the value is not nullish
 */
[null, [], [1]].forEach(arr => {
  if (arr?.length) {
    console.log("%o is not nullish and has one or more items", arr);
  } else {
    console.log("%o is bad", arr);
  }
});

Instead of using

if(data !== null && data !== ''  && data!==undefined) {

// do something
}

You can use below simple code

if(Boolean(value)){ 
// do something 
}

• Values that are intuitively “empty”, like 0, an empty string, null, undefined, and NaN, become false
• Other values become true

There are just a few revisions I would make.

First, always use === instead of == in Javascript. You can read more about that on Stack Overflow.

Second, since undefined is mutable, I would reccomend using

typeof value === "undefined"

instead of

value === undefined

Third, I would remove the !value and value === "" conditions. They are redundant.

My Revision

I would use a slightly different approach than you:

String.isNullOrEmpty = function(value) {
  return !(typeof value === "string" && value.length > 0);
}

This checks if the type of the value is "string" (and thus non-null and not undefined), and if it is not empty. If so, it is not null or empty.

Note that this returns true for non-string inputs, which might not be what you want if you wanted to throw an error for an unexpected input type.

The set of "truthy" and "falsey" values in JavaScript comes from the ToBoolean abstract operation defined in the ECMAScript spec, which is used when coercing a value to a boolean:

+--------------------------------------------------------------------------+
| Argument Type | Result                                                   |
|---------------+----------------------------------------------------------|
| Undefined     | false                                                    |
|---------------+----------------------------------------------------------|
| Null          | false                                                    |
|---------------+----------------------------------------------------------|
| Boolean       | The result equals the input argument (no conversion).    |
|---------------+----------------------------------------------------------|
| Number        | The result is false if the argument is +0, −0, or NaN;   |
|               | otherwise the result is true.                            |
|---------------+----------------------------------------------------------|
| String        | The result is false if the argument is the empty String  |
|               | (its length is zero); otherwise the result is true.      |
|---------------+----------------------------------------------------------|
| Object        | true                                                     |
+--------------------------------------------------------------------------+

From this table, we can see that null and undefined are both coerced to false in a boolean context. However, your fields.length === 0 does not map generally onto a false value. If fields.length is a string, then it will be treated as false (because a zero-length string is false), but if it is an object (including an array) it will coerce to true.

If fields should be a string, then !fields is a sufficient predicate. If fields is an array, your best check might be:

if (!fields || fields.length === 0)