一週內被兩位同事問到幾乎相同的問題，這一定是天意！趕緊寫篇FAQ以免天公伯不開心～
【問題】
- 用JavaScript要怎麼實現Dictionary<string, T>？
- JavaScript有沒有類似LINQ Where()、Select()、OrderBy()的東西？
回答第一個問題，JavaScript物件本身就具備Dictionary<string, T>的特性，範例如下：
<!DOCTYPEhtml>
<html>
<head>
<metacharset="utf-8">
<metaname="viewport"content="width=device-width">
<title>JS Dictionary<string, T></title>
</head>
<body>
<scriptsrc="https://code.jquery.com/jquery-1.12.4.js"></script>
<script>
function Player(Id, Name, RegDate, Score) {this.Id = Id;this.Name = Name;this.RegDate = RegDate;this.Score = Score;}
var p1 = new Player("P1", "Jeffrey", new Date(1900, 0, 1), 32767);
var p2 = new Player("P2", "Darkthread", new Date(2016, 6, 2), 65536);
//使用JavaScript物件模擬Dictionary<string, Player>var dict = {};//加入或指定key為P1的內容 dict["P1"] = p1; dict["P2"] = p2;//讀取key為P1的項目console.log("P1.Name = " + dict["P1"].Name);
//檢查key是否存在if (dict["P1"]) console.log("P1存在");
if (!dict["Q1"]) console.log("Q1不存在");
//模擬Dictionary<string, T>.Keys//IE7、8相容var keys = [];for (var key in dict) keys.push(key);
console.log(keys);
//IE9+及其他瀏覽器console.log(Object.keys(dict));
//模擬Dictionary<string, T>.Valuesvar values = $.map(keys, function(key) { return dict[key] });
console.log("values[0].Name=" + values[0].Name); console.log("values[1].Name=" + values[1].Name);//移除指定key值項目 delete dict["P1"];if (!dict["P1"]) console.log("P1已移除");
</script>
</body>
</html>
執行結果：Live Demo
"P1.Name = Jeffrey""P1存在""Q1不存在" ["P1", "P2"] ["P1", "P2"]"P1已移除""P1.Name = Jeffrey""P1存在""Q1不存在" ["P1", "P2"] ["P1", "P2"]"values[0].Name=Jeffrey""values[1].Name=Darkthread""P1已移除"
補充，在TypeScript如要宣告Dictionary<string, Player>強型別，寫法為var dict: { ["key": string]: Player } = {};
問題二，JavaScript能否做到LINQ Where() Select() OrderBy()的效果？
類似需求我慣用jQuery.grep()、jQuery.map()搞定，排序則可用JavaScript Array本身的sort()方法，但sort()會改掉陣列本身的順序，若要比照OrderBy()的效果，得先用.slice(0)另建一個複本，示範如下：
<!DOCTYPEhtml>
<html>
<head>
<metacharset="utf-8">
<metaname="viewport"content="width=device-width">
<title>JS LINQ Where(), Select() and OrderBy()</title>
</head>
<body>
<scriptsrc="https://code.jquery.com/jquery-1.12.4.js"></script>
<script>
function Player(Id, Name, RegDate, Score) {this.Id = Id;this.Name = Name;this.RegDate = RegDate;this.Score = Score;}
var p1 = new Player("P1", "Jeffrey", new Date(1900, 0, 1), 32767);
var p2 = new Player("P2", "Darkthread", new Date(2016, 6, 2), 65536);
//用陣列模擬List<Player>var list = [p1, p2];//Where(o => o.Score > 255)var res = $.grep(list, function(o) { return o.Score > 255 });
console.log("Where(o => o.Score > 255).Count=" + res.length);//Select(o => new { PlayerId = o.Id, PlayerName = o.Name })var res = $.map(list, function(o) { return { PlayerId: o.Id, PlayerName: o.Name }; });
console.log("Select(o => new { PlayerId = o.Id, PlayerName = o.Name }).Count=" + res.length); console.log("res[0].PlayerId=" + res[0].PlayerId); console.log("res[1].PlayerName=" + res[1].PlayerName);//OrderBy(o => o.Name)//OrderBy不更動List<T>順序，在JavaScript要用slice(0)先複製新陣列物件//以免sort影響原陣列排序var sorted = list.slice(0).sort(function(a, b) {
if (a.Name === b.Name) return 0;
return (a.Name < b.Name) ? -1 : 1;});
console.log("sorted[0].Name=" + sorted[0].Name); console.log("sorted[1].Name=" + sorted[1].Name);//OrderByDecending(o => o.Score) sorted = list.slice(0).sort(function(a, b) {if (a.Score === b.Score) return 0;
return (a.Score > b.Score) ? -1 : 1;});
console.log("sorted[0].Score=" + sorted[0].Score); console.log("sorted[1].Score=" + sorted[1].Score); </script>
</body>
</html>
執行結果：Live Demo
"Where(o => o.Score > 255).Count=2""Select(o => new { PlayerId = o.Id, PlayerName = o.Name }).Count=2""res[0].PlayerId=P1""res[1].PlayerName=Darkthread""sorted[0].Name=Darkthread""sorted[1].Name=Jeffrey""sorted[0].Score=65536""sorted[1].Score=32767"以上範例使用jQuery.grep()模擬Where()、用jQuery.map()模擬Select()，而在ECMAScript 5規格，JavaScript Array已加入foreach、filter、map等方法，可以取代jQuery的.each()、.grep()及.map()，但存在IE7/IE8不支援的限制需要留意。ES5內建的filter()與map()用起來跟jQuery版差不多，換用工程不大，以下為改寫範例： Live Demo
//Where(o => o.Score > 255)var res = list.filter(function(o) { return o.Score > 255 });
console.log("Where(o => o.Score > 255).Count=" + res.length);//Select(o => new { PlayerId = o.Id, PlayerName = o.Name })var res = list.map(function(o) { return { PlayerId: o.Id, PlayerName: o.Name }; });
console.log("Select(o => new { PlayerId = o.Id, PlayerName = o.Name }).Count=" + res.length); console.log("res[0].PlayerId=" + res[0].PlayerId); console.log("res[1].PlayerName=" + res[1].PlayerName);最後，如果你覺得以上做法不夠原汁原味，還是想在JavaScript執行正統的LINQ方法，例如：
var queryResult = Enumerable.From(jsonArray).Where(function (x) { return x.user.id < 200 })
.OrderBy(function (x) { return x.user.screen_name })
.Select(function (x) { return x.user.screen_name + ':' + x.text })
.ToArray();
有個Open Source專案－linq.js可以實現以上夢想。不過，該專案已有段時間未更新，採用前也應該入考量。而依我個人看法，既然在寫前端，不妨改變思維，依循JavaScript風格解決才是王道。既然已有簡便做法能滿足需求，硬要復刻還原過往的習慣，並不利於執行效能及團隊協作，用JavaScript簡便搞定還是較好的選擇。