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深入理解javascript的getTime()方法

 更新時間：2017年02月16日 23:32:35   投稿：mdxy-dxy  
這篇文章主要介紹了深入理解javascript的getTime()方法,需要的朋友可以參考下

1、理解getTime

getTime() 方法返回一個時間的格林威治時間數(shù)值。

可以使用這個方法把一個日期時間賦值給另一個Date 對象。

語法：

dateObj.getTime()
參數(shù)：

無。

返回值：

getTime 方法的返回值一個數(shù)值，表示從1970年1月1日0時0分0秒（UTC，即協(xié)調(diào)世界時）距離該日期對象所代表時間的毫秒數(shù)。

例子：

使用 getTime() 復(fù)制日期對象

創(chuàng)建一個擁有相同時間值的日期對象。

var birthday = new Date(1994, 12, 10);
var copy = new Date();
copy.setTime(birthday.getTime());

測試結(jié)果如下：

image

    既然表示從1970年1月1日開始計(jì)算的毫秒數(shù)，如果小于1970年呢，會是什么情況？（測試結(jié)果如下，是負(fù)數(shù)）

image

2、應(yīng)用場景

2.1 測量代碼執(zhí)行時間

連續(xù)調(diào)用兩個新生成的日期對象的 getTime 方法，根據(jù)兩次調(diào)用的返回值求得時間差。這可以用于計(jì)算某些操作的執(zhí)行時間。

var end, start, i;
start = new Date();
for (i = 0; i < 100000; i++) {
 Math.sqrt(i);
}
end = new Date();

console.log("Operation took " + (end.getTime() - start.getTime()) + " msec");

在chrome瀏覽器中進(jìn)行測試：

image

2.2 甘特圖時間表示

最近在調(diào)整teamwork gantt中，發(fā)現(xiàn)，是一個task任務(wù)項(xiàng)的開始時間與結(jié)束時間，都是用一個整數(shù)表示，而該數(shù)值就是UTC表示法。格式如下：

{"tasks":[
   {"id":-1,"name":"Gantt editor","code":"","level":0,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":21,"end":1399672799999,"startIsMilestone":true,"endIsMilestone":false,"collapsed":false,"assigs":[],"hasChild":true}
   ,{"id":-2,"name":"coding","code":"","level":1,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":10,"end":1398203999999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"description":"","progress":0,"hasChild":true}
   ,{"id":-3,"name":"gantt part","code":"","level":2,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":2,"end":1397167199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-4,"name":"editor part","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1397167200000,"duration":4,"end":1397685599999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-5,"name":"testing","code":"","level":1,"status":"STATUS_SUSPENDED","canWrite":true,"start":1398981600000,"duration":6,"end":1399672799999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","description":"","progress":0,"hasChild":true}
   ,{"id":-6,"name":"test on safari","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1398981600000,"duration":2,"end":1399327199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"8SF:11","hasChild":false}
   ,{"id":-7,"name":"test on safari","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1398981600000,"duration":2,"end":1399327199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"8SF","hasChild":false}
   ,{"id":-8,"name":"test on ie","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1400688000000,"duration":3,"end":1400947199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-9,"name":"test on chrome","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1399327200000,"duration":2,"end":1399499999999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"links":"","hasChild":false}
   ,{"id":-10,"name":"Gantt editor","code":"","level":0,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":21,"end":1399672799999,"startIsMilestone":true,"endIsMilestone":false,"collapsed":false,"assigs":[],"hasChild":true}
   ,{"id":-11,"name":"coding","code":"","level":1,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":10,"end":1398203999999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"description":"","progress":0,"hasChild":true}
   ,{"id":-12,"name":"gantt part","code":"","level":2,"status":"STATUS_ACTIVE","canWrite":true,"start":1396994400000,"duration":2,"end":1397167199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-14,"name":"editor part","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1397167200000,"duration":4,"end":1397685599999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-15,"name":"testing","code":"","level":1,"status":"STATUS_SUSPENDED","canWrite":true,"start":1398981600000,"duration":6,"end":1399672799999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","description":"","progress":0,"hasChild":true}
   ,{"id":-16,"name":"test on safari","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1398981600000,"duration":2,"end":1399327199999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-17,"name":"test on ie","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1399327200000,"duration":3,"end":1399586399999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ,{"id":-18,"name":"test on chrome","code":"","level":2,"status":"STATUS_SUSPENDED","canWrite":true,"start":1399327200000,"duration":2,"end":1399499999999,"startIsMilestone":false,"endIsMilestone":false,"collapsed":false,"assigs":[],"depends":"","hasChild":false}
   ],"selectedRow":0,"canWrite":true,"canWriteOnParent":true}

在甘特圖中，使用getTime表示時間有以下優(yōu)勢：

計(jì)算兩個時間差比較方便
數(shù)據(jù)存儲結(jié)構(gòu)比較簡單
由于gantt涉及到畫圖，使用整型數(shù)據(jù)方便處理

3、瀏覽器的兼容性

使用js要收到瀏覽器的限制，不同瀏覽器對js類庫的支持強(qiáng)弱也不同。而作為標(biāo)準(zhǔn)類庫Date的一個方法，其適應(yīng)的瀏覽器如下：

image

image

4、參考資料

Date.prototype.getTime()：https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Date/getTime

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