悟透前端 | 參悟Javascript中的call和apply

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於前端工程師來說，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":"算是常用的函數方法，允許通過函數和在函數調用中指定","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":"的指向。那麼這兩個方法到底有什麼區別呢？本文將詳細介紹這兩個方法，順便加深對其理解。","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"call","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"方法使用一個指定的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":" 值和單獨給出的一個或多個參數來調用一個函數。允許爲不同的對象分配和調用屬於一個對象的函數/方法。提供新的 ","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"this ","attrs":{}},{"type":"text","text":"值給當前調用的函數/方法。你可以使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":" 來實現繼承：寫一個方法，然後讓另外一個新的對象來繼承它（而不是在新對象中再寫一次這個方法）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"const 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使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":"， 可以只寫一次這個方法然後在另一個對象中繼承它，而不用在新對象中重複寫該方法。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"語法","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"func.apply(thisArg, 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5 開始可以使用類數組對象。 瀏覽器兼容性 請參閱本文底部內容。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"返回值","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"調用有指定","attrs":{}},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":"值和參數的函數的結果。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"call和apply的區別","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"它們的作用一模一樣，區別僅在於傳入參數形式的不同。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5c/5c13fa338c1f57e0bbf2ec954cbd8c7e.jpeg","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":" 接受兩個參數，第一個參數指定了函數體內 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":" 對象的指向，第二個參數爲一個帶下標的集合，這個集合可以爲數組，也可以爲類數組，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法把這個集合中的元素作爲參數傳遞給調用的函數：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"const func = function (a, b, c) {\n console.log([a, b, c]);\n};\nfunc.apply(null, [1, 2, 3]); // [ 1, 2, 3 ]","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這段代碼中，參數 1、2、3 被放在數組中一起傳入 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"func","attrs":{}}],"attrs":{}},{"type":"text","text":" 函數，它們分別對應 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"func","attrs":{}}],"attrs":{}},{"type":"text","text":" 參數列表中的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"a 、b 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的參數在內部就是用一個數組來表示的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從這個意義上說，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":" 比 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":" 的使用率更高，不關心具體多少參數被傳入函數，只要用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":" 一股腦地推過去就可以了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":" 是包裝在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":" 上的一顆語法糖，如果明確地知道函數接收多少個參數，而且想一目瞭然地表達形參和實參的對應關係，那麼可以用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":" 來傳送參數。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當使用 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true","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果是在嚴格模式下，函數體內的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":" 還是爲 null ：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"const func = function (a, b, c) {\n console.log(this === window);\n};\nfunc.apply(null, [1, 2, 3]); // true","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"改變this指向","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"const obj = {\n site: \"https://www.infoq.com\",\n};\nfunction func() {\n console.log(this.site);\n}\nfunc.call(obj); // https://www.infoq.com","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"call","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法的第一個參數是作爲函數上下文的對象，上面的代碼把 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"obj","attrs":{}}],"attrs":{}},{"type":"text","text":" 作爲參數傳給了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"func","attrs":{}}],"attrs":{}},{"type":"text","text":"，此時函數裏的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"this","attrs":{}}],"attrs":{}},{"type":"text","text":" 便指向了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"obj","attrs":{}}],"attrs":{}},{"type":"text","text":" 對象。此處 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"func","attrs":{}}],"attrs":{}},{"type":"text","text":" 函數相當於下面的方法：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"const obj = {\n site: \"https://www.infoq.com\",\n};\nfunction func() {\n console.log(obj.site);\n}","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"apply()方法的一些妙用","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Math.max","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":" 可以實現得到數組中最大的一項","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Math.max","attrs":{}}],"attrs":{}},{"type":"text","text":"不支持","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Math.max([param1,param2])","attrs":{}}],"attrs":{}},{"type":"text","text":"也就是數組，但是它支持","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Math.max(param1,param2...)","attrs":{}}],"attrs":{}},{"type":"text","text":"，所以可以根據","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"apply","attrs":{}}],"attrs":{}},{"type":"text","text":"的特點來解決 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"var 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target = new Array(\"1\", \"2\", \"3\");\nconst source = new Array(\"4\", \"5\", \"6\");\nconst result = Array.prototype.push.apply(target, source); // 得到合併後數組的長度，因爲push就是返回一個數組的長度\nconsole.log(target); // [ '1', '2', '3', '4', '5', '6' ]\nconsole.log(source); // [ '4', '5', '6' ]\nconsole.log(result); // 6","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}