JavaScript 中的 !== 是什么？
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What is !== in javascript?
提问by Cr4sh Over
The code is used to verify an image or cell selection. My question is What is !== used for in the following function:
该代码用于验证图像或单元格选择。我的问题是什么是 !== 用于以下函数：
function checkSelected() {
var cellList,
selectedCell,
imgList,
selectedImg,
i
cellList = document.getElementById("imgTable")
cellList = cellList.getElementsByTagName("td")
imgList = document.getElementById("b_list")
imgList = imgList.getElementsByTagName("img")
if (cellList[0].style.border.indexOf("7px outset") !== -1) { selectedCell = cellList[0] }
if (selectedCell === undefined || selectedImg === undefined) { return }
selectedCell.style.backgroundImage = "url(" + selectedImg.src + ")"
selectedCell.firstChild.style.visibility = "hidden"
selectedCell.style.border = "1px solid"
selectedImg.style.border = "1px solid"
}
回答by Vivin Paliath
!==is a stricter inequality that does not perform any type coercion on the operands, compared to !=, which does perform type coercion.
!==是一个更严格的不等式，它不对操作数执行任何类型强制，相比之下!=，它确实执行类型强制。
So !==will return true if the operands are not equal and/or not of the same type.
因此，!==如果操作数不相等和/或类型不同，则返回 true。
In contrast !=returns true if the operands are equal. If the operands are not of the same type, JavaScript will try to convert both operands into a form suitable for comparison. If the operands are objects, then JavaScript will compare their references (memory addresses). This is best demonstrated as follows:
相反，!=如果操作数相等则返回真。如果操作数的类型不同，JavaScript 会尝试将两个操作数转换为适合比较的形式。如果操作数是对象，那么 JavaScript 将比较它们的引用（内存地址）。最好的证明如下：
"3" != 3; // returns false because JavaScript will perform
// type coercion and compare the values
"3" !== 3; // returns true because the first operand is a
// string whereas the second is an integer. Since
// they are of different types, they are not equal.
For more information, take a look at Comparison Operatorson MDN.
欲了解更多信息，看看比较操作上MDN。
回答by Cam
It means "not strictly equal to", as opposed to !=which means "not equal to".
它的意思是“不严格等于”，而不是!=“不等于”的意思。
There are two ways to check for equality: ==and ===, which are "equals" and "strictly equals" respectively. To see the exact difference, check out this table.
有两种检查相等性的方法：==and ===，分别是“相等”和“严格相等”。要查看确切差异，请查看此表。
!=and !==are simply the corresponding negations of those operations. So for example a !== bis the same as !(a === b)
!=并且!==只是这些操作的相应否定。所以例如a !== b与!(a === b)
