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JavaScript - Number

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• JavaScript - Number
  • Number
    • NaN - Not a Number
    • Infinity
    • Numbers as Objects
  • Hexadecimal 十六进制
  • Adding Numbers and Strings

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Number

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

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

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

• JavaScript Numbers are Always 64-bit Floating Point

  • 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

• Integer Precision

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

  • The maximum number of decimals is 17.

• Floating Precision

  • Floating point arithmetic is not always 100% accurate. it helps to multiply and divide:

  let x = (0.2 * 10 + 0.1 * 10) / 10;
  

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NaN - Not a Number

• NaN

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

• NaN is a number:

  • typeof NaN returns number.

• isNaN():

  • the global JavaScript function to find out if a value is a not a number

• Computation:

  • If you use NaN in a mathematical operation, the result will also be NaN or be a concatenation.

var x = 100 / "Apple"; //x is NaN
console.log(x); //NaN

console.log(typeof x); //number
console.log(isNaN(x)); //true

var x = 100 / "10";
console.log(x); //10
console.log(isNaN(x)); //false

var x = NaN;
var y = 5;
console.log(x + y); //NaN

var x = NaN;
var y = "5";
console.log(x + y); //NaN5

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Infinity

• Infinity (or -Infinity)

  • the value JavaScript will return if you calculate a number outside the largest possible number.
• Infinity is a number:
  • • typeof Infinity returns number.

• Division by 0 (zero) also generates Infinity.

• isFinite():

  • the global JavaScript function to find out if a value is Infinity

var x = 2;
while (x != Infinity) {
  // while (isFinite(x)) {
  console.log(x);
  x **= 2;
}
// 2
// 4
// 16
// 256
// 65536
// 4294967296
// 18446744073709552000
// 3.402823669209385e+38
// 1.157920892373162e+77
// 1.3407807929942597e+154

var y = 2 / 0;

console.log(y); //Infinity
console.log(typeof Infinity); //number
console.log(isFinite(y)); //false

console.log(y == Infinity); //true

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Numbers as Objects

• numbers can also be defined as objects with the keyword new.

• Do not create Number objects.

• The new keyword complicates the code and slows down execution speed.

• Number Objects can produce unexpected results

var x = 500;
var y = new Number(500);
var z = new Number(500);

console.log(x == y); //true
console.log(x === y); //false

console.log(y == z); //false
console.log(y === z); //false

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Hexadecimal 十六进制

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

• By default, JavaScript displays numbers as base 10 decimals.

• But you can use the toString() method to output numbers from base 2 to base 36.

• Hexadecimal is base 16. Decimal is base 10. Octal is base 8. Binary is base 2.

var x = 0xff;

console.log(x); //255
console.log(x.toString(2)); //11111111
console.log(x.toString(8)); //377
console.log(x.toString(10)); //255
console.log(x.toString(16)); //ff
console.log(x.toString(32)); //7v

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Adding Numbers and Strings

• + operator for both addition and concatenation.

Operand 1st	Operand 2nd	Result
Number	Number	Number
String	String	Concatenation
Number	String	Concatenation
String	Number	Concatenation

• Numeric Strings

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

let x = "100";
let y = "10";
var z = x / y; //10
var z = x * y; //1000
var z = x - y; //90
var z = x + y; //"10010"

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