• 前頁
• 次頁

這個章節將介紹如何在 JavaScript 中處理數字與日期。

數字(Numbers)

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 -(2^53 -1) 到 2^53 -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， andNaN (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)。

十進制數值

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

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; // 2147483648var FLT_EXPONENT = 0b01111111100000000000000000000000; // 2139095040var FLT_MANTISSA = 0b00000000011111111111111111111111; // 8388607

八進制數值

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; // 493var 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 with0o, e.g.:

var a = 0o10; // ES2015: 8

十六進制數值

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; // 2951479051793528300000x123456789abcdef; // 819855292164869000xa; // 10

指數運算

1e3; // 10002e6; // 20000000.1e2; // 10

Number 物件

The built-inNumber 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 predefinedNumber object as shown above, and not as a property of aNumber object you create yourself.

下面這張表格整理了Number 物件的屬性

Number的屬性

TheNumber prototype provides methods for retrieving information fromNumber objects in various formats. The following table summarizes the methods ofNumber.prototype.

Math 物件

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

Math.PI;

Similarly, standard mathematical functions are methods ofMath. 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 ofMath take arguments in radians.

The following table summarizes theMath object's methods.

Unlike many other objects, you never create aMath object of your own. You always use the built-inMath object.

Date 物件

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。

CallingDate without thenew keyword returns a string representing the current date and time.

Theparameters 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);.

Date 的方法

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

• "set" methods, for setting date and time values inDate objects.
• "get" methods, for getting date and time values fromDate objects.
• "to" methods, for returning string values fromDate objects.
• parse and UTC methods, for parsingDate 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 agetDay method that returns the day of the week, but no correspondingsetDay 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。 ThegetTime method returns the number of milliseconds since January 1, 1970, 00:00:00 for aDate 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 monthendYear.setFullYear(today.getFullYear()); // Set year to this yearvar msPerDay = 24 * 60 * 60 * 1000; // Number of milliseconds per dayvar daysLeft = (endYear.getTime() - today.getTime()) / msPerDay;var daysLeft = Math.round(daysLeft); //returns days left in the year

This example creates aDate object namedtoday that contains today's date. It then creates aDate object namedendYear and sets the year to the current year. Then, using the number of milliseconds per day, it computes the number of days betweentoday andendYear, usinggetTime and rounding to a whole number of days.

Theparse method is useful for assigning values from date strings to existingDate objects. For example, the following code usesparse andsetTime to assign a date value to theIPOdate object:

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

範例

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."。

• 前頁
• 次頁