數字與日期

這個章節將介紹如何在 JavaScript 中處理數字與日期。

在 JavaScript 中, Number所使用的標準依照 double-precision 64-bit binary format IEEE 754 (i.e. number的區間是 -(2⁵³ -1) 到 2⁵³ -1)。整數是沒有特定的類型。

此外還可以顯示浮點數，三種符號數值: +Infinity， -Infinity， and NaN (not-a-number)。

BigInt 是Javascript最新的功能，它可以表示一個很大的整數。使用 BigInt需要注意一點，BigInt 和Number不能在同一個operation混用還有當用 Math 物件時不能使用BigInt。

請參照 JavaScript data types and structures 來取得更多詳細資料。

你可以用四種進制表示數字：十進制 (decimal)，二進制 (binary)，八進制 (octal) 以及十六進制 (hexadecimal)。

1234567890
42

// 以零為開頭時要小心：

0888 // 888 解析為 十進制數值
0777 // 在 non-strict 模式下將解析成八進制 (等同於十進制的 511)

請注意，十進位數字允許第一個數字設為零(0)的話，前提是後面接的數字必須要有一個數字大於8(例如輸入0888結果會是888，輸入068結果會是68)，不然則會被轉成８進位(例如0777結果會是511，輸入063結果會是51)。

二進制數值以 0 為開頭並跟著一個大寫或小寫的英文字母 「B」 (0b 或 0B)。如果 0b 後面接著的數字不是 0 或 1，那會丟出 SyntaxError(語法錯誤): "Missing binary digits after 0b"。

var FLT_SIGNBIT  = 0b10000000000000000000000000000000; // 2147483648
var FLT_EXPONENT = 0b01111111100000000000000000000000; // 2139095040
var FLT_MANTISSA = 0B00000000011111111111111111111111; // 8388607

八進制數值以 0 為開頭。如果 0 後面的數字超出 0 到 7 這個範圍，將會被解析成十進制數值。

var n = 0755; // 493
var m = 0644; // 420

Strict mode in ECMAScript 5 forbids octal syntax. Octal syntax isn't part of ECMAScript 5, but it's supported in all browsers by prefixing the octal number with a zero: 0644 === 420 and"\045" === "%". In ECMAScript 2015, octal numbers are supported if they are prefixed with 0o, e.g.: 

var a = 0o10; // ES2015: 8

十六進制數值以 0 為開頭並跟著一個大寫或小寫的英文字母 「X」(0x 或 0X)。如果 0b 後面接著的值超出範圍 (0123456789ABCDEF)，那會丟出 SyntaxError(語法錯誤):"Identifier starts immediately after numeric literal"。

0xFFFFFFFFFFFFFFFFF // 295147905179352830000
0x123456789ABCDEF   // 81985529216486900
0XA                 // 10

1E3   // 1000
2e6   // 2000000
0.1e2 // 10

The built-in Number object has properties for numerical constants, such as maximum value, not-a-number, and infinity. You cannot change the values of these properties and you use them as follows:

var biggestNum = Number.MAX_VALUE;
var smallestNum = Number.MIN_VALUE;
var infiniteNum = Number.POSITIVE_INFINITY;
var negInfiniteNum = Number.NEGATIVE_INFINITY;
var notANum = Number.NaN;

You always refer to a property of the predefined Number object as shown above, and not as a property of a Number object you create yourself.

下面這張表格整理了 Number 物件的屬性

Number 的屬性

The Number prototype provides methods for retrieving information from Number objects in various formats. The following table summarizes the methods of Number.prototype.

The built-in Math object has properties and methods for mathematical constants and functions. For example, the Math object's PI property has the value of pi (3.141...), which you would use in an application as

Math.PI

Similarly, standard mathematical functions are methods of Math. These include trigonometric, logarithmic, exponential, and other functions. For example, if you want to use the trigonometric function sine, you would write

Math.sin(1.56)

Note that all trigonometric methods of Math take arguments in radians.

The following table summarizes the Math object's methods.

Unlike many other objects, you never create a Math object of your own. You always use the built-in Math object.

JavaScript 沒有所謂日期(date)的數據型態(data type)。你可以使用 Date 物件及其方法去設定日期跟時間來滿足你的需求 。Date 物件有大量的設定取得操作日期的方法(method)，但它沒有屬性。

JavaScript 處理日期的方式跟Java類似。這兩個語言有許多一樣的date方法，且都將日期儲存為從1970年1月1號0時0分0秒以來的毫秒數(millisecond)。

Date 物件範圍是 -100,000,000 days to 100,000,000 days 以1970年1月1號0時0分0秒UTC為基準。

創建一個Date物件:

var dateObjectName = new Date([parameters]);

在這裡創建一個名為dateObjectName 的 Date 物件；它可以是一個新的物件或是某個以存在的物件當中的屬性。

Calling Date without the new keyword returns a string representing the current date and time.

The parameters in the preceding syntax can be any of the following:

• Nothing: creates today's date and time. For example, today = new Date();.
• A string representing a date in the following form: "Month day, year hours:minutes:seconds." For example, var Xmas95 = new Date("December 25, 1995 13:30:00"). If you omit hours, minutes, or seconds, the value will be set to zero.
• A set of integer values for year, month, and day. For example, var Xmas95 = new Date(1995, 11, 25).
• A set of integer values for year, month, day, hour, minute, and seconds. For example, var Xmas95 = new Date(1995, 11, 25, 9, 30, 0);.

The Date object methods for handling dates and times fall into these broad categories:

• "set" methods, for setting date and time values in Date objects.
• "get" methods, for getting date and time values from Date objects.
• "to" methods, for returning string values from Date objects.
• parse and UTC methods, for parsing Date strings.

With the "get" and "set" methods you can get and set seconds, minutes, hours, day of the month, day of the week, months, and years separately. There is a getDay method that returns the day of the week, but no corresponding setDay method, because the day of the week is set automatically. These methods use integers to represent these values as follows:

• Seconds and minutes: 0 到 59
• Hours: 0 到 23
• Day: 0 (星期日) 到 6 (星期六)
• Date: 1 到 31 (這個月的第幾天)
• Months: 0 (一月) 到 11 (十二月)
• Year: years since 1900

舉例，假設你定義了一個日期如下：

var Xmas95 = new Date('December 25, 1995');

那 Xmas95.getMonth() 將會回傳 11， Xmas95.getFullYear() 會回傳 1995。

getTime 及 setTime 這兩個方法對於比較日期有幫助。 The getTime method returns the number of milliseconds since January 1, 1970, 00:00:00 for a Date object.

For example, the following code displays the number of days left in the current year:

var today = new Date();
var endYear = new Date(1995, 11, 31, 23, 59, 59, 999); // Set day and month
endYear.setFullYear(today.getFullYear()); // Set year to this year
var msPerDay = 24 * 60 * 60 * 1000; // Number of milliseconds per day
var daysLeft = (endYear.getTime() - today.getTime()) / msPerDay;
var daysLeft = Math.round(daysLeft); //returns days left in the year

This example creates a Date object named today that contains today's date. It then creates a Date object named endYear and sets the year to the current year. Then, using the number of milliseconds per day, it computes the number of days between today and endYear, using getTime and rounding to a whole number of days.

The parse method is useful for assigning values from date strings to existing Date objects. For example, the following code uses parse and setTime to assign a date value to the IPOdate object:

var IPOdate = new Date();
IPOdate.setTime(Date.parse('Aug 9, 1995'));

下面這個範例，JSClock() 這個函式將會以數位時鐘的格式回傳時間。

function JSClock() {
  var time = new Date();
  var hour = time.getHours();
  var minute = time.getMinutes();
  var second = time.getSeconds();
  var temp = '' + ((hour > 12) ? hour - 12 : hour);
  if (hour == 0)
    temp = '12';
  temp += ((minute < 10) ? ':0' : ':') + minute;
  temp += ((second < 10) ? ':0' : ':') + second;
  temp += (hour >= 12) ? ' P.M.' : ' A.M.';
  return temp;
}

 JSClock 這個函式會先建立一個名為 time 的 Date 物件; 因為沒有提供任何引數，所以會放入目前的日期及時間。接著呼叫 getHours 、 getMinutes 以及 getSeconds 這三個方法將現在的時、分以及秒分別指定給 hour 、 minute 以及 second 這三個變數。

接著的四行指令將會建立一個時間的字串。第一行的指令建立了一個變數 temp，以條件運算式指定值; 如果 hour 大於 12，那就指定 (hour - 12)，不然會直接指定 hour， 但如果 hour 等於 0 ， 則改為 12。

接著下一行將 minute 加到 temp 中。如果 minute 小於 10, 則會在附加時補上一個零; 不然的話會直接加上冒號及分鐘數。秒數也是以同樣的作法附加到 temp 上。

最後，判斷 hour 是不是大於等於 12 ，如果是就在 temp 加上 "P.M." ，不然就加上 "A.M."。