Learn JS #16

JavaScript Numbers

JavaScript has only one type of number. Numbers can be written with or without decimals.


Example

let x = 3.14;    // A number with decimals
let y = 3;       // A number without decimals

Extra large or extra small numbers can be written with scientific (exponent) notation:

Example

let x = 123e5;    // 12300000
let y = 123e-5;   // 0.00123

JavaScript Numbers are Always 64-bit Floating Point

Unlike many other programming languages, JavaScript does not define different types of numbers, like integers, short, long, floating-point etc.

JavaScript numbers are always stored as double precision floating point numbers, following the international IEEE 754 standard.

This format stores numbers in 64 bits, where the number (the fraction) is stored in bits 0 to 51, the exponent in bits 52 to 62, and the sign in bit 63:

Value (aka Fraction/Mantissa) Exponent Sign
52 bits (0 – 51) 11 bits (52 – 62) 1 bit (63)

Integer Precision

Integers (numbers without a period or exponent notation) are accurate up to 15 digits.

Example

let x = 999999999999999;   // x will be 999999999999999
let y = 9999999999999999;  // y will be 10000000000000000

The maximum number of decimals is 17.


Floating Precision

Floating point arithmetic is not always 100% accurate:

let x = 0.2 + 0.1;

To solve the problem above, it helps to multiply and divide:

let x = (0.2 * 10 + 0.1 * 10) / 10;

Adding Numbers and Strings

WARNING !!

JavaScript uses the + operator for both addition and concatenation.

Numbers are added. Strings are concatenated.

If you add two numbers, the result will be a number:

Example

let x = 10;
let y = 20;
let z = x + y;

If you add a number and a string, the result will be a string concatenation:

Example

let x = 10;
let y = "20";
let z = x + y;

If you add a string and a number, the result will be a string concatenation:

Example

let x = "10";
let y = 20;
let z = x + y;

A common mistake is to expect this result to be 30:

Example

let x = 10;
let y = 20;
let z = "The result is: " + x + y;

A common mistake is to expect this result to be 102030:

Example

let x = 10;
let y = 20;
let z = "30";
let result = x + y + z;

The JavaScript interpreter works from left to right.

First 10 + 20 is added because x and y are both numbers.

Then 30 + “30” is concatenated because z is a string.


Numeric Strings

JavaScript strings can have numeric content:

let x = 100;         // x is a number

let y = "100";       // y is a string

JavaScript will try to convert strings to numbers in all numeric operations:

This will work:

let x = "100";
let y = "10";
let z = x / y;

This will also work:

let x = "100";
let y = "10";
let z = x * y;

And this will work:

let x = "100";
let y = "10";
let z = x - y;

But this will not work:

let x = "100";
let y = "10";
let z = x + y;

In the last example JavaScript uses the + operator to concatenate the strings.


NaN – Not a Number

NaN is a JavaScript reserved word indicating that a number is not a legal number.

Trying to do arithmetic with a non-numeric string will result in NaN (Not a Number):

Example

let x = 100 / "Apple";

However, if the string contains a numeric value , the result will be a number:

Example

let x = 100 / "10";

You can use the global JavaScript function isNaN() to find out if a value is a not a number:

Example

let x = 100 / "Apple";
isNaN(x);

Watch out for NaN. If you use NaN in a mathematical operation, the result will also be NaN:

Example

let x = NaN;
let y = 5;
let z = x + y;

Or the result might be a concatenation like NaN5:

Example

let x = NaN;
let y = "5";
let z = x + y;

NaN is a number: typeof NaN returns number:

Example

typeof NaN;

Infinity

Infinity (or -Infinity) is the value JavaScript will return if you calculate a number outside the largest possible number.

Example

let myNumber = 2;
// Execute until Infinity
while (myNumber != Infinity) {
  myNumber = myNumber * myNumber;
}

Division by 0 (zero) also generates Infinity:

Example

let x =  2 / 0;
let y = -2 / 0;

Infinity is a number: typeof Infinity returns number.

Example

typeof Infinity;

Hexadecimal

JavaScript interprets numeric constants as hexadecimal if they are preceded by 0x.

Example

let x = 0xFF;

Example

let myNumber = 32;
myNumber.toString(32);
myNumber.toString(16);
myNumber.toString(12);
myNumber.toString(10);
myNumber.toString(8);
myNumber.toString(2);

JavaScript Numbers as Objects

Normally JavaScript numbers are primitive values created from literals:

let x = 123;

But numbers can also be defined as objects with the keyword new:

let y = new Number(123);

Example

let x = 123;
let y = new Number(123);

When using the == operator, x and y are equal:

let x = 500;
let y = new Number(500);

When using the === operator, x and y are not equal.

let x = 500;
let y = new Number(500);

Note the difference between (x==y) and (x===y).

(x == y) true or false?

let x = new Number(500);
let y = new Number(500);

(x === y) true or false?

let x = new Number(500);
let y = new Number(500);

77 thoughts on “Learn JS #16

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