在JavaScript模擬C# Dictionary、LINQ Where、Select與OrderBy
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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:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>JS Dictionary<string, T></title>
</head>
<body>
<script src="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>.Values
var 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:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>JS LINQ Where(), Select() and OrderBy()</title>
</head>
<body>
<script src="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。
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