Why does != work and just = doesn't?

This question already has an answer here:

Which equals operator (== vs ===) should be used in JavaScript comparisons? 50 answers

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Your condition is actually an assignment:

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

You should be using == :

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

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The = is an assignment operation.

The != is an inequality operator.

The == is an equality operator.

I guess what you need is the == operator. So replace your code with:

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

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JS Comparison operators

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

For example:

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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