This question already has an answer here: Detecting an undefined object property 40 answers How to check a not-defined variable in JavaScript 11 answers How to handle 'undefined' in javascript [duplicate] 3 answers how to check if a variable exist in javascript? 8 answers If you are interested in finding out whether a variable has been declared regardless of its value, then usi
这个问题在这里已经有了答案： 检测未定义的对象属性40个答案 如何在JavaScript中检查未定义的变量11个答案 如何在javascript中处理'undefined'[复制] 3个答案 如何检查一个变量是否存在于JavaScript中？ 8个答案 如果您有兴趣了解变量是否已被声明而不管其值如何，那么使用in运算符是最安全的方法。 考虑这个例子。 // global scope var theFu; // theFu has been declared, but its value is undefined ty
This question already has an answer here: Which equals operator (== vs ===) should be used in JavaScript comparisons? 50 answers The === operator ensures that not only the values are equal, but the two items being compared are of the same type too; Whereas the == operator only checks that the values of the two items are equal As @amnotiam mentioned in the comments, you may also want to ch
这个问题在这里已经有了答案： 应该在JavaScript比较中使用哪个等于运算符（== vs ===）？ 50个答案 ===运算符确保不仅值相等，而且比较的两个项目也是相同的类型; ==运算符只检查两个项目的值是否相等 正如评论中提到的@amnotiam，你可能也想看看抽象平等比较算法 ===用于检查类型值 var str="300"; //this gt execute if(str==="300") { alert("this alert get executed"); } //this not execute if(str=
Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? Difference between == and === in JavaScript I have two variables to compare. Result should not be equal, in which condition i need to use != and !== ? because when i use both operator it is working properly, but i need to know exactly what is the difference. Human readable text about their differen
可能重复： Javascript === vs ==：这与我使用的“平等”运算符有关吗？ JavaScript和==和===之间的区别 我有两个变量要比较。 结果不应该相等，在哪种情况下我需要使用！=和！==？ 因为当我使用这两个操作符它正常工作，但我需要确切知道有什么区别。 关于他们的差异的人类可读文本 使用!==和===会比== / !=做更严格的比较。 前者将检查被比较的对象是否具有相同的类型，以及值是否匹配。 使用==将使隐式转换成
Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? As the title states; when should you use the === operator check when using JavaScript, and when not to. Edit: more complete answer found here. Thanks to Mark Byers for pointing it out. _L It is strict type equality operator. It not only checks whether two are equal in value but also of the same ty
可能重复： Javascript === vs ==：这与我使用的“平等”运算符有关吗？ 正如标题所述; 什么时候应该在使用JavaScript时使用===运算符检查，何时不使用。 编辑：在这里找到更完整的答案。 感谢Mark Byers指出。 _L 它是严格的类型相等运算符。 它不仅检查两个值是否相等，而且是相同类型 。 考虑一下比较数字或字符串的情况： if (4 === 4) // same value and type { // true } 但 if (4 == "4") // same value
This question already has an answer here: Which equals operator (== vs ===) should be used in JavaScript comparisons? 50 answers Your condition is actually an assignment: if (document.getElementById("hiddenButton").style.visibility = "hidden") { You should be using == : if (document.getElementById("hiddenButton").style.visibility == "hidden") { The = is an assignment operation. The !=
这个问题在这里已经有了答案： 应该在JavaScript比较中使用哪个等于运算符（== vs ===）？ 50个答案 你的条件实际上是一项任务： if (document.getElementById("hiddenButton").style.visibility = "hidden") { 你应该使用== ： if (document.getElementById("hiddenButton").style.visibility == "hidden") { 该=是一个赋值操作。 !=是一个不等式运算符。 ==是一个相等运算符。 我猜你需要的是==运算符。 所以
Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? When would JavaScript == make more sense than ===? What is the difference between below methods in comparing a string with undefined value. var x; if(x==undefined) { alert(x); } and if(x===undefined) { alert(x); } Why should i prefer second method in this case.. Please let me know advan
可能重复： Javascript === vs ==：这与我使用的“平等”运算符有关吗？ JavaScript ==何时比===更有意义？ 将字符串与未定义的值进行比较时，下面的方法有什么区别？ var x; if(x==undefined) { alert(x); } 和 if(x===undefined) { alert(x); } 为什么我应该在这种情况下更喜欢第二种方法..请让我知道优点.. ==在测试前尝试将值转换为相同的类型，如果它们相同。 "5" == 5 ===不这样做;
Possible Duplicates: Difference between == and === in JavaScript Javascript === vs == : Does it matter which “equal” operator I use? What's the difference between == and === ? Also between !== and !== ? There are lots of answers to this question on Stackoverflow already. Short: == only compares values === compares values + type var check1 = '10', check2 = 10; check1 == ch
可能重复： JavaScript和==和===之间的区别 Javascript === vs ==：这与我使用的“平等”运算符有关吗？ ==和===什么区别？ 也在!==和!== ？ 有很多Stackoverflow上的这个问题的答案。 短： ==只比较值 ===比较值+类型 var check1 = '10', check2 = 10; check1 == check2 // true check1 === check2 // false “==”表示等于，而“===”表示相同。 总之，“==”会在比较时尝试强制/转换值的类型，所以“2”== 2，
Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? What are the differences between === and ==, !== and ==... when should you use one and when should you use the other? Matt === is the Identity operator, and is used to test that value and type are equal. so.. "3" == 3 // true "3" === 3 // false 1 == true // true 1 === true // false "1" == true // tr
可能重复： Javascript === vs ==：这与我使用的“平等”运算符有关吗？ ===和==，！==和==之间有什么区别...什么时候应该使用一个，什么时候应该使用另一个？ 马特 ===是身份运算符，用于测试该值和类型是否相等。 所以.. "3" == 3 // true "3" === 3 // false 1 == true // true 1 === true // false "1" == true // true "1" === true // false 所以当你关心这个值和类型是否相等时，或者不等于使用Identity运算符=
Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? Looking into the answer of Chris Brandsma in Advanced JavaScript Interview Questions what is === in Javascript. If possible please provide a simple example === is the strict equal operator. It only returns a Boolean True if both the operands are equal and of the same type. If a is 2, and b is 4, a =
可能重复： Javascript === vs ==：这与我使用的“平等”运算符有关吗？ 在高级JavaScript面试中寻找Chris Brandsma的答案在Javascript中，什么是=== 。 如果可能请提供一个简单的例子 ===是严格相等的运算符。 如果两个操作数相同且类型相同，它只返回布尔值True。 如果a是2，b是4， a === 2 (True) b === 4 (True) a === '2' (False) 对于以下所有情况， a == 2 a == "2" 2 == '2' ===是'严格平等运营商'
Possible Duplicate: JavaScript === vs == : Does it matter which “equal” operator I use? I asked another question here and received a great answer as follows: $(document).on("keydown", function (e) { if (e.which === 8 && !$(e.target).is("input, textarea") || $(e.target).is('[readonly]')) { e.preventDefault(); } }); Notice the three equal signs === in the if-statement. I h
可能重复： JavaScript === vs ==：这与我使用的“平等”运算符有关吗？ 我在这里问了另一个问题，并得到了一个很好的答案如下： $(document).on("keydown", function (e) { if (e.which === 8 && !$(e.target).is("input, textarea") || $(e.target).is('[readonly]')) { e.preventDefault(); } }); 注意if语句中的三个等号=== 。 我一直以为你只需要两个等号==为javascript / jQuery if语句。 这三个