is this a correct conditional?

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You need to do every comparison completely, like this:

if($_SERVER['REQUEST_URI'] == 'index' || $_SERVER['REQUEST_URI'] == 'post' || $_SERVER['REQUEST_URI'] == 'about'){
  // why this not work for me?
}

Since, what you're doing now, is comparing $_SERVER["REQUEST_URI"] to true :

var_dump("hello" || "index"); // bool(true)

So the comparison is always true, because you're using only two equal signs, which is a loose comparison. Had you used === , the result would be false because a string is not of the same type as a boolean .

Another way is to use an array and in_array() , which will check if a value is in the values of an array:

$values = ["index", "post", "about"];
if (in_array($_SERVER["REQUEST_URI"], $values)) {

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In JS, you're effectively doing this:

if (miString == 'hello') {

That is to say that the logical OR operators will give you the first truthy value, which in this case is 'hello' , and so only that is used for the comparison.

JS doesn't have an operator shorthand for multiple comparisons. You can use multiple == operations, a switch statement, or some construct that searches items in a collection.

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你会想要尝试这样的事情：

var miString = 'hello'; 
if(miString == 'hello' || miString == 'hola' || miString == 'ciao' || miString == 'merhaba'){
 alert('Welcome');
}

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