Built-in Constants

Operations that return NaN

Mathematical operations on values other than numbers return NaN.

"a" + 1
"b" * 3
"cde" - "e"
[1, 2, 3] * 2

An exception: Single-number arrays.

[2] * [3]  // Returns 6

Also, remember that the + operator concatenates strings.

"a" + "b"  // Returns "ab"

Dividing zero by zero returns NaN.

0 / 0         // NaN

Note: In mathematics generally (unlike in JavaScript programming), dividing by zero is not possible.

Math library functions that return NaN

Generally, Math functions that are given non-numeric arguments will return NaN.

Math.floor("a")

The square root of a negative number returns NaN, because Math.sqrt does not support imaginary or complex numbers.

Math.sqrt(-1)

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.

6

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:

  1. If Type(number) is not Number, return false.
  2. If number is NaN, return true.
  3. 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

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

var a = null;

a === null; // true

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 undefined === 'undefined'
  • null is an object that indicates that a variable has been explicitly assigned "no 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
      
  • Accessing the value of a property that doesn't exist
    • let foo = { a: 'a' };
      console.log('is undefined?', foo.b === undefined);
      // is undefined? true
      
  • The return value of a function that doesn't return a value
    • function foo() { return; }
      console.log('is undefined?', foo() === undefined);
      // is undefined? true
      
  • The value of a function argument that is declared but has been omitted from the function call
    • function foo(param) { 
        console.log('is undefined?', param === undefined);
      }
      foo('a');
      foo();
      // is undefined? false
      // is undefined? true
      

undefined is also a property of the global window object.

// Only in browsers
console.log(window.undefined); // undefined
window.hasOwnProperty('undefined'); // true    
5

Before ECMAScript 5 you could actually change the value of the window.undefined property to any other value potentially breaking everything.

Infinity and -Infinity

1 / 0; // Infinity
// Wait! WHAAAT?

Infinity is a property of the global object (therefore a global variable) that represents mathematical infinity. It is a reference to Number.POSITIVE_INFINITY

It is greater than any other value, and you can get it by dividing by 0 or by evaluating the expression of a number that's so big that overflows. This actually means there is no division by 0 errors in JavaScript, there is Infinity!

There is also -Infinity which is mathematical negative infinity, and it's lower than any other value.

To get -Infinity you negate Infinity, or get a reference to it in Number.NEGATIVE_INFINITY.

- (Infinity); // -Infinity

Now let's have some fun with examples:

Infinity > 123192310293; // true
-Infinity < -123192310293; // true
1 / 0; // Infinity
Math.pow(123123123, 9123192391023); // Infinity
Number.MAX_VALUE * 2; // Infinity
23 / Infinity; // 0
-Infinity; // -Infinity
-Infinity === Number.NEGATIVE_INFINITY; // true
-0; // -0 , yes there is a negative 0 in the language
0 === -0; // true
1 / -0; // -Infinity
1 / 0 === 1 / -0; // false
Infinity + Infinity; // Infinity

var a = 0, b = -0;

a === b; // true
1 / a === 1 / b; // false

// Try your own!

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 instead.

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

Number constants

The Number constructor has some built in constants that can be useful

Number.MAX_VALUE;          // 1.7976931348623157e+308
Number.MAX_SAFE_INTEGER;   // 9007199254740991

Number.MIN_VALUE;          // 5e-324
Number.MIN_SAFE_INTEGER;   // -9007199254740991

Number.EPSILON;            // 0.0000000000000002220446049250313

Number.POSITIVE_INFINITY;  // Infinity
Number.NEGATIVE_INFINITY;  // -Infinity

Number.NaN;                // NaN

In many cases the various operators in Javascript will break with values outside the range of (Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER)

Note that Number.EPSILON represents the different between one and the smallest Number greater than one, and thus the smallest possible difference between two different Number values. One reason to use this is due to the nature of how numbers are stored by JavaScript see