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Javascript unable to handle input from an HTML input tag

I am having trouble getting my javascript code to take an input from my HTML input tag.

The basic function of the program is for the user to enter a number, and the program tells the user if their number is positive, negative, or zero.

Here is my code:

<!DOCTYPE html>
<html lang="en">
<head>

    <script type="text/javascript">
        window.onload = function () {

            if (Math.sign("number") = 1) { document.getElementById("demo").innerHTML = "Your number is positive"; }

            else if (Math.sign("number") = -1) { document.getElementById("demo").innerHTML = "Your number is negative"; }

            else { document.getElementById("demo").innerHTML = "Your number is zero"; }
        }
    </script>
</head>

<body>

        <form>
            <input type="number" type="button" id="number">
            <input type="submit" value="submit">
        </form>


        <p id="demo"></p>

</body>

</html>

Answer

function myFunction(){ 

    let number = document.getElementById("number").value
    if (number > 0) { document.getElementById("demo").innerHTML = "Your number is positive"; }
    else if (number < 0) { document.getElementById("demo").innerHTML = "Your number is negative"; }
    else { document.getElementById("demo").innerHTML = "Your number is zero"; }
}      
     <input type="number" type="button" id="number" />
     <button onclick="myFunction()">Submit </button>
  
     <p id="demo"></p>