Numbers

Number values are assigned to variables without quotation marks:

var birthYear = 1982; //1,982--xx
    var price = 16.99;  // decimal places（小數位） must be separated by a dot.
    var temperature = -5;
    
    var bigNumber = 2e45; 
    /* large numbers in Javascript can be represented by the exponential notation. 
    // This example means 2 x 1045 */

Numbers are used for all sorts(排序) of calculations, from simple math operations to complex expressions.

• Basic Math Operations

var num1 = 20;
    var num2 = 3;

    var sum = num1 + num2;
    var subtraction = num1 - num2;
    var division = num1 / num2;
    var multiplication = num1 * num2;

    var average = (num1 + num2) / 2;

    /*
先乘除後加減
    括號可用於形成表達式。在這種情況下，它用於定義the order of the operations.。沒有括號，除法會在求和之前完成，
    導致不正確的平均值。
    */

• The Math Object

Built-in Math object

var power = Math.pow(2,4);  /*  2的四次方, which is 16. */

    var roundedNumber = Math.round(2.7); // 四捨五入Rounds to the nearest integer. Result: 3

    var roundUp = Math.ceil(4.3) //無條件進位 Rounds to the next integer. Result: 5
    
    var roundDown = Math.floor(4.6) // 小於等於所給數字的最大整數。Rounds to the current integer. Result: 4

    var squareRoot = Math.sqrt(36) // 開根號 Square root. Result: 6

    var randomNumber = Math.random() // 0~1 小數點亂數 The result is a random number between 0 and 1
    var randomNumber = Math.random()＊100 // 0~100 小數點亂數 The result is a random number between 0 and 100
    
Methods of the number objectvar Math.round(Math.random()＊100)//// 0~100 整數亂數 

• Methods of the number object

Every data type is an object and has its own methods.

var myNumber = 3.78633;
    // toFixed()=修改小數點後幾位
    // toFixed() rounds it to a fixed number of decimal places

    console.log( myNumber.toFixed(2) ); // will show 3.79
    console.log( myNumber.toFixed(4) ); // will show 3.7863

Check out more number methods here

• Incremental operators（增量）

//方法一
var myNumber = 20;
    myNumber = myNumber + 5;
    console.log(myNumber); // The result will be 25
//This structure variable = variable + value can be simplified with the incremental operator (+=)

//精簡作法
myNumber = 1;
    myNumber += 5;
    console.log(myNumber); // The result will be 6

    // We can also use *=, /= e -= for incremental multiplication, division and subtraction

//可同樣用於字串
var name = "John";
    name += " ";
    name += "Doe";

    console.log(name); // The console will show "John Doe"

• Type conversions（類型轉換）