Built-in Constants

null

null is used for representing the intentional absence of an object value and is a primitive value. Unlike undefined, it is not a property of the global object.

It is equal to undefined but not identical to it.

null == undefined; // true
null === undefined; // false

CAREFUL: The typeof null is 'object'.

typeof null; // 'object';

To properly check if a value is null, compare it with the strict equality operator

var a = null;

a === null; // true

Testing for NaN using isNaN()

`window.isNaN()`

The global function isNaN() can be used to check if a certain value or expression evaluates to NaN. This function (in short) first checks if the value is a number, if not tries to convert it (*), and then checks if the resulting value is NaN. For this reason, this testing method may cause confusion.

^{(*) The “conversion” method is not that simple, see ECMA-262 18.2.3 for a detailed explanation of the algorithm.}

These examples will help you better understand the isNaN() behavior:

isNaN(NaN);          // true
isNaN(1);            // false: 1 is a number
isNaN(-2e-4);        // false: -2e-4 is a number (-0.0002) in scientific notation
isNaN(Infinity);     // false: Infinity is a number
isNaN(true);         // false: converted to 1, which is a number
isNaN(false);        // false: converted to 0, which is a number
isNaN(null);         // false: converted to 0, which is a number
isNaN("");           // false: converted to 0, which is a number
isNaN(" ");          // false: converted to 0, which is a number
isNaN("45.3");       // false: string representing a number, converted to 45.3
isNaN("1.2e3");      // false: string representing a number, converted to 1.2e3
isNaN("Infinity");   // false: string representing a number, converted to Infinity
isNaN(new Date);     // false: Date object, converted to milliseconds since epoch
isNaN("10$");        // true : conversion fails, the dollar sign is not a digit
isNaN("hello");      // true : conversion fails, no digits at all
isNaN(undefined);    // true : converted to NaN
isNaN();             // true : converted to NaN (implicitly undefined)
isNaN(function(){}); // true : conversion fails
isNaN({});           // true : conversion fails
isNaN([1, 2]);       // true : converted to "1, 2", which can't be converted to a number

This last one is a bit tricky: checking if an Array is NaN. To do this, the Number() constructor first converts the array to a string, then to a number; this is the reason why isNaN([]) and isNaN([34]) both return false, but isNaN([1, 2]) and isNaN([true]) both return true: because they get converted to "", "34", "1,2" and "true" respectively. In general, an array is considered NaN by isNaN() unless it only holds one element whose string representation can be converted to a valid number.

`Number.isNaN()`

In ECMAScript 6, the Number.isNaN() function has been implemented primarily to avoid the problem of window.isNaN() of forcefully converting the parameter to a number. Number.isNaN(), indeed, doesn’t try to convert the value to a number before testing. This also means that only values of the type number, that are also NaN, return true (which basically means only Number.isNaN(NaN)).

From ECMA-262 20.1.2.4:

When the Number.isNaN is called with one argument number, the following steps are taken:

- If Type(number) is not Number, return `false`. - If number is `NaN`, return `true`. - Otherwise, return `false`.

Some examples:

// The one and only
Number.isNaN(NaN);          // true

// Numbers
Number.isNaN(1);            // false
Number.isNaN(-2e-4);        // false
Number.isNaN(Infinity);     // false

// Values not of type number
Number.isNaN(true);         // false
Number.isNaN(false);        // false
Number.isNaN(null);         // false
Number.isNaN("");           // false
Number.isNaN(" ");          // false
Number.isNaN("45.3");       // false
Number.isNaN("1.2e3");      // false
Number.isNaN("Infinity");   // false
Number.isNaN(new Date);     // false
Number.isNaN("10$");        // false
Number.isNaN("hello");      // false
Number.isNaN(undefined);    // false
Number.isNaN();             // false
Number.isNaN(function(){}); // false
Number.isNaN({});           // false
Number.isNaN([]);           // false
Number.isNaN([1]);          // false
Number.isNaN([1, 2]);       // false
Number.isNaN([true]);       // false

NaN

NaN stands for “Not a Number.” When a mathematical function or operation in JavaScript cannot return a specific number, it returns the value NaN instead.

It is a property of the global object, and a reference to Number.NaN

window.hasOwnProperty('NaN'); // true
NaN; // NaN

Perhaps confusingly, NaN is still considered a number.

typeof NaN; // 'number'

Don’t check for NaN using the equality operator. See isNaN instead.

NaN == NaN  // false
NaN === NaN // false

undefined and null

At first glance it may appear that null and undefined are basically the same, however there are subtle but important differences.

undefined is the absence of a value in the compiler, because where it should be a value, there hasn’t been put one, like the case of an unassigned variable.

`undefined` is a global value that represents the absence of an assigned value.

typeof null === 'object'

Setting a variable to undefined means the variable effectively does not exist. Some processes, such as JSON serialization, may strip undefined properties from objects. In contrast, null properties indicate will be preserved so you can explicitly convey the concept of an “empty” property.

The following evaluate to undefined:

A variable when it is declared but not assigned a value (i.e. defined)
- ```
let foo;
console.log('is undefined?', foo === undefined);
// is undefined? true
```
- ```
let foo = { a: 'a' };
console.log('is undefined?', foo.b === undefined);
// is undefined? true
```
- ```
function foo() { return; }
console.log('is undefined?', foo() === undefined);
// is undefined? true
```
- ```
function foo(param) {
  console.log('is undefined?', param === undefined);
}
foo('a');
foo();
// is undefined? false
// is undefined? true
```

Built-in Constants

null

Testing for NaN using isNaN()

`window.isNaN()`

`Number.isNaN()`

NaN

undefined and null

Infinity and -Infinity

Operations that return NaN

Math library functions that return NaN

Number constants