path: root/files/zh-tw/web/javascript/guide/numbers_and_dates/index.html

---
title: 數字與日期
slug: Web/JavaScript/Guide/Numbers_and_dates
translation_of: Web/JavaScript/Guide/Numbers_and_dates
---
<div>{{jsSidebar("JavaScript Guide")}} {{PreviousNext("Web/JavaScript/Guide/Expressions_and_Operators", "Web/JavaScript/Guide/Text_formatting")}}</div>

<p class="summary">這個章節將介紹如何在 JavaScript 中處理數字與日期。</p>

<h2 id="數字Numbers">數字(Numbers)</h2>

<p>在 JavaScript 中, Number所使用的標準依照 <a class="external external-icon" href="https://en.wikipedia.org/wiki/Double-precision_floating-point_format">double-precision 64-bit binary format IEEE 754</a> (i.e. number的區間是 -(2<sup>53</sup> -1) 到 2<sup>53</sup> -1)。<strong>整數是沒有特定的類型</strong>。</p>

<p>此外還可以顯示浮點數，三種符號數值: <code>+</code>{{jsxref("Infinity")}}， <code>-</code>{{jsxref("Infinity")}}， and {{jsxref("NaN")}} (not-a-number)。</p>

<p>{{jsxref("BigInt")}} 是Javascript最新的功能，它可以表示一個很大的整數。使用 <code>BigInt需要注意一點</code>，<code>BigInt</code> 和{{jsxref("Number")}}不能在同一個operation混用還有當用 {{jsxref("Math")}} 物件時不能使用<code>BigInt</code>。</p>

<p>請參照 <a href="/en-US/docs/Web/JavaScript/Data_structures">JavaScript data types and structures</a> 來取得更多詳細資料。</p>

<p>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)。</p>

<h3 id="十進制數值">十進制數值</h3>

<pre class="brush: js">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)
</pre>

<p>請注意，十進位數字允許第一個數字設為零(<code>0</code>)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)。</p>

<h3 id="二進制數值">二進制數值</h3>

<p>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」 (<code>0b</code> 或 <code>0B</code>)。如果 <code>0b</code> 後面接著的數字不是 0 或 1，那會丟出 <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError">SyntaxError(語法錯誤)</a></code>: "Missing binary digits after 0b"。</p>

<pre class="brush: js">var FLT_SIGNBIT  = 0b10000000000000000000000000000000; // 2147483648
var FLT_EXPONENT = 0b01111111100000000000000000000000; // 2139095040
var FLT_MANTISSA = 0B00000000011111111111111111111111; // 8388607</pre>

<h3 id="八進制數值">八進制數值</h3>

<p>八進制數值以 0 為開頭。如果 <code>0</code> 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。</p>

<pre class="brush: js">var n = 0755; // 493
var m = 0644; // 420
</pre>

<p>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: <code>0644 === 420</code> and<code>"\045" === "%"</code>. In ECMAScript 2015, octal numbers are supported if they are prefixed with <code>0o</code>, e.g.: </p>

<pre class="brush: js">var a = 0o10; // ES2015: 8
</pre>

<h3 id="十六進制數值">十六進制數值</h3>

<p>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」(<code>0x</code> 或 <code>0X</code>)。如果 <code>0b</code> 後面接著的值超出範圍 (0123456789ABCDEF)，那會丟出 <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError">SyntaxError(語法錯誤)</a></code>:"Identifier starts immediately after numeric literal"。</p>

<pre class="brush: js">0xFFFFFFFFFFFFFFFFF // 295147905179352830000
0x123456789ABCDEF   // 81985529216486900
0XA                 // 10
</pre>

<h3 id="指數運算">指數運算</h3>

<pre class="brush: js">1E3   // 1000
2e6   // 2000000
0.1e2 // 10</pre>

<h2 id="Number_物件"><code>Number</code> 物件</h2>

<p>The built-in {{jsxref("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:</p>

<pre class="brush: js">var biggestNum = Number.MAX_VALUE;
var smallestNum = Number.MIN_VALUE;
var infiniteNum = Number.POSITIVE_INFINITY;
var negInfiniteNum = Number.NEGATIVE_INFINITY;
var notANum = Number.NaN;
</pre>

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

<p>下面這張表格整理了 <code>Number</code> 物件的屬性</p>

<p><code style="font-weight: 700;">Number</code><strong style="font-style: inherit; font-weight: 700;"> 的屬性</strong></p>

<table class="standard-table">
 <thead>
  <tr>
   <th scope="col">屬性</th>
   <th scope="col">描述</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{jsxref("Number.MAX_VALUE")}}</td>
   <td>可表示的最大數值</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.MIN_VALUE")}}</td>
   <td>可表示的最小數值</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.NaN")}}</td>
   <td>表示「非數值」（Not-A-Number）的數值</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.NEGATIVE_INFINITY")}}</td>
   <td>Special negative infinite value; returned on overflow</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.POSITIVE_INFINITY")}}</td>
   <td>Special positive infinite value; returned on overflow</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.EPSILON")}}</td>
   <td>Difference between one and the smallest value greater than one that can be represented as a {{jsxref("Number")}}.</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.MIN_SAFE_INTEGER")}}</td>
   <td>可以在 JavaScript 中安全表示的最小數值。</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.MAX_SAFE_INTEGER")}}</td>
   <td>可以在 JavaScript 中安全表示的最大數值。</td>
  </tr>
 </tbody>
</table>

<table class="standard-table">
 <caption><code>Number</code> 的方法</caption>
 <thead>
  <tr>
   <th>方法</th>
   <th>描述</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{jsxref("Number.parseFloat()")}}</td>
   <td>字串轉換成浮點數。<br>
    等同於全域函式 {{jsxref("parseFloat", "parseFloat()")}} 。</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.parseInt()")}}</td>
   <td>以指定的基數將字串轉換成整數。<br>
    等同於全域函式 {{jsxref("parseInt", "parseInt()")}} 。</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.isFinite()")}}</td>
   <td>判定給定的值是不是一個有限數。</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.isInteger()")}}</td>
   <td>判定給定的值是不是一個整數</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.isNaN()")}}</td>
   <td>Determines whether the passed value is {{jsxref("Global_Objects/NaN", "NaN")}}. More robust version of the original global {{jsxref("Global_Objects/isNaN", "isNaN()")}}.</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.isSafeInteger()")}}</td>
   <td>Determines whether the provided value is a number that is a <dfn>safe integer</dfn>.</td>
  </tr>
 </tbody>
</table>

<p>The <code>Number</code> prototype provides methods for retrieving information from <code>Number</code> objects in various formats. The following table summarizes the methods of <code>Number.prototype</code>.</p>

<table class="standard-table">
 <caption><code>Number.prototype</code> 的方法</caption>
 <thead>
  <tr>
   <th scope="col">方法</th>
   <th scope="col">描述</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{jsxref("Number.toExponential", "toExponential()")}}</td>
   <td>Returns a string representing the number in exponential notation.</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.toFixed", "toFixed()")}}</td>
   <td>Returns a string representing the number in fixed-point notation.</td>
  </tr>
  <tr>
   <td>{{jsxref("Number.toPrecision", "toPrecision()")}}</td>
   <td>Returns a string representing the number to a specified precision in fixed-point notation.</td>
  </tr>
 </tbody>
</table>

<h2 id="Math_物件"><code>Math</code> 物件</h2>

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

<pre class="brush: js">Math.PI
</pre>

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

<pre class="brush: js">Math.sin(1.56)
</pre>

<p>Note that all trigonometric methods of <code>Math</code> take arguments in radians.</p>

<p>The following table summarizes the <code>Math</code> object's methods.</p>

<table class="standard-table">
 <caption><code>Math</code> 的方法</caption>
 <thead>
  <tr>
   <th scope="col">方法</th>
   <th scope="col">描述</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{jsxref("Math.abs", "abs()")}}</td>
   <td>絕對值</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.sin", "sin()")}}, {{jsxref("Math.cos", "cos()")}}, {{jsxref("Math.tan", "tan()")}}</td>
   <td>三角函數; 引數以弳度表示</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.asin", "asin()")}}, {{jsxref("Math.acos", "acos()")}}, {{jsxref("Math.atan", "atan()")}}, {{jsxref("Math.atan2", "atan2()")}}</td>
   <td>反三角函數; 回傳值以弳度表示</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.sinh", "sinh()")}}, {{jsxref("Math.cosh", "cosh()")}}, {{jsxref("Math.tanh", "tanh()")}}</td>
   <td>雙曲函數; 引數以 hyperbolic angle 表示</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.asinh", "asinh()")}}, {{jsxref("Math.acosh", "acosh()")}}, {{jsxref("Math.atanh", "atanh()")}}</td>
   <td>反雙曲函數; 回傳值以 hyperbolic angle 表示</td>
  </tr>
  <tr>
   <td>
    <p>{{jsxref("Math.pow", "pow()")}}, {{jsxref("Math.exp", "exp()")}}, {{jsxref("Math.expm1", "expm1()")}}, {{jsxref("Math.log10", "log10()")}}, {{jsxref("Math.log1p", "log1p()")}}, {{jsxref("Math.log2", "log2()")}}</p>
   </td>
   <td>指數及對數函式</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.floor", "floor()")}}, {{jsxref("Math.ceil", "ceil()")}}</td>
   <td>回傳小於等於/大於等於指定數字的最大/最小整數</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.min", "min()")}}, {{jsxref("Math.max", "max()")}}</td>
   <td>Returns lesser or greater (respectively) of comma separated list of numbers arguments</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.random", "random()")}}</td>
   <td>回傳一個介於 0 到 1 之間的數值</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.round", "round()")}}, {{jsxref("Math.fround", "fround()")}}, {{jsxref("Math.trunc", "trunc()")}},</td>
   <td>Rounding and truncation functions.</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.sqrt", "sqrt()")}}, {{jsxref("Math.cbrt", "cbrt()")}}, {{jsxref("Math.hypot", "hypot()")}}</td>
   <td>Square root, cube root, Square root of the sum of square arguments.</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.sign", "sign()")}}</td>
   <td>The sign of a number, indicating whether the number is positive, negative or zero.</td>
  </tr>
  <tr>
   <td>{{jsxref("Math.clz32", "clz32()")}},<br>
    {{jsxref("Math.imul", "imul()")}}</td>
   <td>Number of leading zero bits in the 32-bit binary representation.<br>
    The result of the C-like 32-bit multiplication of the two arguments.</td>
  </tr>
 </tbody>
</table>

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

<h2 id="Date_物件"><code>Date</code> 物件</h2>

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

<p>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)。</p>

<p><code>Date</code> 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。</p>

<p>創建一個<code>Date</code>物件:</p>

<pre class="brush: js">var dateObjectName = new Date([parameters]);
</pre>

<p>在這裡創建一個名為<code>dateObjectName</code> 的 <code>Date</code> 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。</p>

<p>Calling <code>Date</code> without the <code>new</code> keyword returns a string representing the current date and time.</p>

<p>The <code>parameters</code> in the preceding syntax can be any of the following:</p>

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

<h3 id="Date_的方法"> <code>Date</code> 的方法</h3>

<p>The <code>Date</code> object methods for handling dates and times fall into these broad categories:</p>

<ul>
 <li>"set" methods, for setting date and time values in <code>Date</code> objects.</li>
 <li>"get" methods, for getting date and time values from <code>Date</code> objects.</li>
 <li>"to" methods, for returning string values from <code>Date</code> objects.</li>
 <li>parse and UTC methods, for parsing <code>Date</code> strings.</li>
</ul>

<p>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 <code>getDay</code> method that returns the day of the week, but no corresponding <code>setDay</code> method, because the day of the week is set automatically. These methods use integers to represent these values as follows:</p>

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

<p>舉例，假設你定義了一個日期如下：</p>

<pre class="brush: js">var Xmas95 = new Date('December 25, 1995');
</pre>

<p>那 <code>Xmas95.getMonth()</code> 將會回傳 11， <code>Xmas95.getFullYear()</code> 會回傳 1995。</p>

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

<p>For example, the following code displays the number of days left in the current year:</p>

<pre class="brush: js">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
</pre>

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

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

<pre class="brush: js">var IPOdate = new Date();
IPOdate.setTime(Date.parse('Aug 9, 1995'));
</pre>

<h3 id="範例">範例</h3>

<p>下面這個範例，<code>JSClock()</code> 這個函式將會以數位時鐘的格式回傳時間。</p>

<pre class="brush: js">function JSClock() {
  var time = new Date();
  var hour = time.getHours();
  var minute = time.getMinutes();
  var second = time.getSeconds();
  var temp = '' + ((hour &gt; 12) ? hour - 12 : hour);
  if (hour == 0)
    temp = '12';
  temp += ((minute &lt; 10) ? ':0' : ':') + minute;
  temp += ((second &lt; 10) ? ':0' : ':') + second;
  temp += (hour &gt;= 12) ? ' P.M.' : ' A.M.';
  return temp;
}
</pre>

<p> <code>JSClock</code> 這個函式會先建立一個名為 <code>time</code> 的 <code>Date</code> 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 <code>getHours</code> 、 <code>getMinutes</code> 以及 <code>getSeconds</code> 這三個方法將現在的時、分以及秒分別指定給 <code>hour</code> 、 <code>minute</code> 以及 <code>second</code> 這三個變數。</p>

<p>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 <code>temp</code>，以條件運算式指定值; 如果 <code>hour</code> 大於 12，那就指定 (<code>hour - 12</code>)，不然會直接指定 <code>hour</code>， 但如果 <code>hour</code> 等於 0 ， 則改為 12。</p>

<p>接著下一行將 <code>minute</code> 加到 <code>temp</code> 中。如果 <code>minute</code> 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 <code>temp</code> 上。</p>

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

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