條件運算子- JavaScript

JavaScript Demo: Expressions - Conditional operator. xxxxxxxxxx. 1. function getFee(isMember) {. 2. return (isMember ? '$2.00' : '$10.00');. SkiptomaincontentSkiptosearchSkiptoselectlanguage給開發者的網頁技術文件JavaScriptJavaScript參考文件運算式與運算子條件運算子ArticleActions正體中文(繁體)ThispagewastranslatedfromEnglishbythecommunity.LearnmoreandjointheMDNWebDocscommunity.嘗試一下語法描述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 AggregateErrorArrayArrayBufferAsyncFunctionAtomicsBigIntBigInt64ArrayBigUint64ArrayBooleanDataViewDatedecodeURI()decodeURIComponent()encodeURI()encodeURIComponent()Error Deprecated escape()eval()EvalErrorFinalizationRegistryFloat32ArrayFloat64ArrayFunctionGeneratorGeneratorFunctionglobalThisInfinityInt16ArrayInt32ArrayInt8ArrayInternalErrorIntlisFinite()isNaN()JSONMapMathNaNNumberObjectparseFloat()parseInt()PromiseProxyRangeErrorReferenceErrorReflectRegExpSetSharedArrayBufferStringSymbolSyntaxErrorTypedArrayTypeErrorUint16ArrayUint32ArrayUint8ArrayUint8ClampedArrayundefined Deprecated unescape()URIErrorWeakMapWeakRefWeakSetWebAssembly Expressions&operators 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Misc JavaScripttechnologiesoverview Lexicalgrammar JavaScriptdatastructures Enumerabilityandownershipofproperties Iterationprotocols Strictmode Transitioningtostrictmode Templateliterals Deprecatedfeatures 嘗試一下語法描述規範瀏覽器相容性參見條件運算子嘗試一下 條件(三元)運算子是JavaScript唯一用到三個運算元的運算子：在一個條件後面會跟著一個問號(?)，如果條件是truthy，在冒號(:)前的表達式會被執行，如果條件是falsy，在冒號後面的表達式會被執行，這個運算子常常被用來當作if的簡潔寫法. 語法condition?exprIfTrue:exprIfFalse參數 condition 值用來做為條件的表達式 exprIfTrue 如果condition的值是truthy(等於或是可轉換為true),exprIfTrue會被執行 exprIfFalse 如果condition的值是falsy(等於或是可轉換為false),exprIfFalse會被執行 描述除了false,可能是falsy的表達式有null,NaN,0,空字串("")和undefined.如果condition是他們其中之一,那麼條件表達式的結果會是exprIfFalse的執行結果. 一個簡單的範例: varage=26; varbeverage=(age>=21)?"Beer":"Juice"; console.log(beverage);//"Beer" 一個常用來處理null的用法: functiongreeting(person){ varname=person?person.name:"stranger"; return"Howdy,"+name; } console.log(greeting({name:'Alice'}));//"Howdy,Alice" console.log(greeting(null));//"Howdy,stranger" 條件鏈條件(三元)運算子是右相依性的(right-associative),代表他可以以下面的方式鏈結,類似於if…elseif …elseif…else的鏈結方法: functionexample(…){ returncondition1?value1 :condition2?value2 :condition3?value3 :value4; } //Equivalentto: functionexample(…){ if(condition1){returnvalue1;} elseif(condition2){returnvalue2;} elseif(condition3){returnvalue3;} else{returnvalue4;} } 規範SpecificationECMAScriptLanguageSpecification#sec-conditional-operator瀏覽器相容性BCDtablesonlyloadinthebrowser參見 ifstatement Nullishcoalescingoperator Optionalchaining Makingdecisionsinyourcode—conditionals Expressionsandoperators Foundaproblemwiththispage?EditonGitHubSourceonGitHubReportaproblemwiththiscontentonGitHubWanttofixtheproblemyourself?SeeourContributionguide.Lastmodified:2022年6月30日,byMDNcontributors