为什么！=工作而只是=不？

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

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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") {

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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。 所以用你的代码替换：

if (document.getElementById("hiddenButton").style.visibility == "hidden") {

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JS比较运算符

==      is equal to 
===     is exactly equal to (value and type)
!=      is not equal

例如：

var x = 1;  //define and assigned and now x equal to 1
x = 3;        //now x equal to 3

if( x == 4) {
    //you won't see this alert
    alert('Hello, x is 4 now');
} else {
    //you will see this alert
    alert('Hello, x hasn not been changed and it is still ' + x.toString());
}

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