getter - JavaScript - MDN Web Docs
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最少要有零個參數(請參見Incompatible ES5 change: literal getter and setter functions must now have exactly zero or one arguments 的詳細資料) ...
SkiptomaincontentSkiptoselectlanguage給開發者的網頁技術文件JavaScriptJavaScript參考文件FunctionsgetterArticleActions正體中文(繁體)ThispagewastranslatedfromEnglishbythecommunity.LearnmoreandjointheMDNWebDocscommunity.Tryit語法敘述示例規範瀏覽器相容性參見RelatedTopics
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Warning:-file-isbeingassigneda//#sourceMappingURL,butalreadyhasoneTypeError:invalidArray.prototype.sortargumentWarning:08/09isnotalegalECMA-262octalconstantRangeError:radixmustbeanintegerSyntaxError:invalidregularexpressionflag"x"SyntaxError:returnnotinfunctionTypeError:X.prototype.ycalledonincompatibletypeReferenceError:can'taccesslexicaldeclaration`X'beforeinitializationTypeError:can'taccessproperty"x"of"y"TypeError:can'tassigntoproperty"x"on"y":notanobjectTypeError:can'tdefineproperty"x":"obj"isnotextensibleTypeError:property"x"isnon-configurableandcan'tbedeletedTypeError:can'tredefinenon-configurableproperty"x"TypeError:cyclicobjectvalueTypeError:can'taccessdeadobjectSyntaxError:applyingthe'delete'operatortoanunqualifiednameisdeprecatedReferenceError:deprecatedcallerorargumentsusageWarning:expressionclosuresaredeprecatedSyntaxError:"0"-prefixedoctalliteralsandoctalescapeseq.aredeprecatedSyntaxError:Using//@toindicatesourceURLpragmasisdeprecated.Use//#insteadWarning:String.xisdeprecated;useString.prototype.xinsteadWarning:Date.prototype.toLocaleFormatisdeprecatedSyntaxError:testforequality(==)mistypedasassignment(=)?TypeError:settinggetter-onlyproperty"x"SyntaxError:Unexpected'#'usedoutsideofclassbodySyntaxError:identifierstartsimmediatelyafternumericliteralSyntaxError:illegalcharacterTypeError:cannotuse'in'operatortosearchfor'x'in'y'RangeError:invalidarraylengthReferenceError:invalidassignmentleft-handsideTypeError:invalidassignmenttoconst"x"RangeError:invaliddateSyntaxError:for-inloopheaddeclarationsmaynothaveinitializersSyntaxError:adeclarationintheheadofafor-ofloopcan'thaveaninitializerTypeError:invalid'instanceof'operand'x'TypeError:'x'isnotiterableSyntaxError:JSON.parse:badparsingSyntaxError:MalformedformalparameterURIError:malformedURIsequenceSyntaxError:missing]afterelementlistSyntaxError:missing:afterpropertyidSyntaxError:missing}afterfunctionbodySyntaxError:missing}afterpropertylistSyntaxError:missingformalparameterSyntaxError:missing=inconstdeclarationSyntaxError:missingnameafter.operatorSyntaxError:missing)afterargumentlistSyntaxError:missing)afterconditionSyntaxError:missing;beforestatementTypeError:MoreargumentsneededRangeError:repeatcountmustbenon-negativeTypeError:"x"isnotanon-nullobjectTypeError:"x"hasnopropertiesSyntaxError:missingvariablenameTypeError:can'tdeletenon-configurablearrayelementRangeError:argumentisnotavalidcodepointTypeError:"x"isnotaconstructorTypeError:"x"isnotafunctionReferenceError:"x"isnotdefinedRangeError:precisionisoutofrangeError:Permissiondeniedtoaccessproperty"x"TypeError:"x"isread-onlySyntaxError:redeclarationofformalparameter"x"TypeError:ReduceofemptyarraywithnoinitialvalueSyntaxError:"x"isareservedidentifierRangeError:repeatcountmustbelessthaninfinityWarning:unreachablecodeafterreturnstatementSyntaxError:"usestrict"notallowedinfunctionwithnon-simpleparametersInternalError:toomuchrecursionReferenceError:assignmenttoundeclaredvariable"x"ReferenceError:referencetoundefinedproperty"x"SyntaxError:UnexpectedtokenTypeError:"x"is(not)"y"SyntaxError:functionstatementrequiresanameSyntaxError:unterminatedstringliteral
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Tryit語法敘述示例規範瀏覽器相容性參見getterTryit
get語法會將物件屬性,綁定到屬性被檢索時,所呼叫的函式。
語法{getprop(){...}}
{get[expression](){...}}參數
prop
要綁定到給定函式的名稱。
expression
自ECMAScript2015開始,可以用計算屬性名稱(computedpropertyname),綁定到給定函式。
敘述有時候,物件的屬性可能需要回傳動態數值、或要在不使用明確的方法呼叫下,反映出內部變數的狀態。
在JavaScript可以用getter達到這個目的。
儘管可以用getter與setter的關聯建立虛擬屬性的類型,但getter無法被綁定到同時擁有實際數值的屬性。
使用get語法時,請注意以下情況:
可以擁有一個以數字或字串為代表的標示符;
最少要有零個參數(請參見IncompatibleES5change:literalgetterandsetterfunctionsmustnowhaveexactlyzerooronearguments的詳細資料)
不能以有另一個get的objectliteral、或相同屬性入口(dataentry)的data形式出現(不能使用{getx(){},getx(){}}and{x:...,getx(){}})。
getter可以用delete(en-US)操作符移除。
示例在物件初始器(objectinitializers)內定義新物件的getter這程式碼將給obj物件建立虛擬屬性latest,它會回傳log陣列的最後一個單元。
varobj={
log:['example','test'],
getlatest(){
if(this.log.length==0)returnundefined;
returnthis.log[this.log.length-1];
}
}
console.log(obj.latest);//"test".
請注意latest不會因為數值被指派而改變。
使用delete操作符移除getter如果想移除getter,可以使用delete(en-US)完成之:
deleteobj.latest;
使用defineProperty給現有物件定義getter若想在任何時候給現有物件添增getter,請使用Object.defineProperty()。
varo={a:0};
Object.defineProperty(o,'b',{get:function(){returnthis.a+1;}});
console.log(o.b)//Runsthegetter,whichyieldsa+1(whichis1)使用計算屬性名varexpr='foo';
varobj={
get[expr](){return'bar';}
};
console.log(obj.foo);//"bar"Smart/self-overwriting/lazygettersGetter提供了定義物件屬性的方法,但它本身並不會去自動計算,直到想訪問它。
除非需要用getter,否則數值計算會被延緩;如果不需要用到getter,那就永遠無須支付計算的開銷。
針對屬性值lazy或delay、並暫存以留作未來訪問的最佳化技巧稱作smart或memoizedgetter:初次計算時會呼叫getter、接著被暫存以便在不重算的情況下做後續訪問。
這種技巧在以下情況會派上用場:
如果數值開銷很昂貴(例如需要大量RAM或CPU時間、產生worker執行緒、檢索遠端文件等)
如果現在並不需要數值:可能是現在用不到、或在某些情況下完全用不到。
如果使用的話,該數值會被訪問數次、且該數值永遠不會更改、或不應該更改。
也就是說,出於getter不會重新計算的理由,不要針對數值預期會改變的屬性,使用lazygetter。
下例的物件擁有作為自己的屬性的getter。
在取得該屬性後,它會從物件被移除、並以隱式數值屬性重新增加、最後回傳之。
getnotifier(){
deletethis.notifier;
returnthis.notifier=document.getElementById('bookmarked-notification-anchor');
},
針對Firefox程式碼,另請參見定義defineLazyGetter()函式的XPCOMUtils.jsm程式模塊。
get與defineProperty在使用classes時,儘管get關鍵字與Object.defineProperty()會出現相同結果,但其中有微妙的差異。
在使用get時,屬性會在物件的原型被定義;而在使用Object.defineProperty()時,屬性會在被套用的實例內定義。
classExample{
gethello(){
return'world';
}
}
constobj=newExample();
console.log(obj.hello);
//"world"
console.log(Object.getOwnPropertyDescriptor(obj,'hello'));
//undefined
console.log(Object.getOwnPropertyDescriptor(Object.getPrototypeOf(obj),'hello'));
//{configurable:true,enumerable:false,get:functiongethello(){return'world';},set:undefined}規範
規範
狀態
註解
ECMAScript5.1(ECMA-262)Thedefinitionof'ObjectInitializer'inthatspecification.
Standard
初始定義。
ECMAScript2015(6thEdition,ECMA-262)Thedefinitionof'Methoddefinitions'inthatspecification.
Standard
增加計算屬性名。
ECMAScript(ECMA-262)Thedefinitionof'Methoddefinitions'inthatspecification.
LivingStandard
瀏覽器相容性BCDtablesonlyloadinthebrowser參見
setter
delete(en-US)
Object.defineProperty()
__defineGetter__(en-US)
__defineSetter__(en-US)
JavaScript教學的定義Getter與Setter
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