建構子- JavaScript - MDN Web Docs

constructor（建構子）是個隨著class 一同建立並初始化物件的特殊方法。

SkiptomaincontentSkiptoselectlanguage給開發者的網頁技術文件JavaScriptJavaScript參考文件Classes建構子ArticleActions正體中文(繁體)ThispagewastranslatedfromEnglishbythecommunity.LearnmoreandjointheMDNWebDocscommunity.語法敘述示例規範瀏覽器相容性參見RelatedTopics JavaScript Tutorials: Completebeginners JavaScriptbasics JavaScriptfirststeps JavaScriptbuildingblocks IntroducingJavaScriptobjects JavaScriptGuide Introduction Grammarandtypes Controlflowanderrorhandling Loopsanditeration Functions Expressionsandoperators Numbersanddates Textformatting Regularexpressions Indexedcollections Keyedcollections Workingwithobjects Detailsoftheobjectmodel Usingpromises Iteratorsandgenerators Metaprogramming JavaScriptmodules Intermediate Client-sideJavaScriptframeworks Client-sidewebAPIs Are-introductiontoJavaScript JavaScriptdatastructures Equalitycomparisonsandsameness Closures Advanced Inheritanceandtheprototypechain Strictmode JavaScripttypedarrays MemoryManagement ConcurrencymodelandEventLoop References: Built-inobjects 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Misc JavaScripttechnologiesoverview Lexicalgrammar JavaScriptdatastructures Enumerabilityandownershipofproperties Iterationprotocols Strictmode Transitioningtostrictmode Templateliterals Deprecatedfeatures 語法敘述示例規範瀏覽器相容性參見建構子 constructor（建構子）是個隨著class一同建立並初始化物件的特殊方法。

語法constructor([arguments]){...}敘述一個class只能有一個稱為constructor的特殊物件。

如果一個class出現兩次以上的constructor，就會發生SyntaxError錯誤。

如果不指定建構子，就會使用預設的建構子。

示例使用constructor方法這段程式碼是從classessample擷取而來。

（線上範例） classSquareextendsPolygon{ constructor(length){ //我們在這裡呼叫了class的建構子提供多邊形的長寬值 super(length,length); //注意：在derivedclass中，super()必須在使用this以前被呼叫。

不這樣的話會發生錯誤。

this.name='Square'; } getarea(){ returnthis.height*this.width; } setarea(value){ this.area=value; } }預設的建構子如上文所說：如果不指定建構子，就會使用預設的建構子。

對baseclasses而言，預設的建構子長得像這樣： constructor(){} 對derivedclass而言，預設的建構子長得像這樣： constructor(...args){ super(...args); }規範SpecificationECMAScriptLanguageSpecification#sec-static-semantics-constructormethod瀏覽器相容性BCDtablesonlyloadinthebrowser參見 super() classexpression(en-US) classdeclaration(en-US) Classes Foundaproblemwiththispage?EditonGitHubSourceonGitHubReportaproblemwiththiscontentonGitHubWanttofixtheproblemyourself?SeeourContributionguide.Lastmodified:Feb15,2022,byMDNcontributors