Onclick event with if condition in javascript html

pretty new to coding. But I have a small assignment where I just not get what function I should do or how I should write it down. Basicly it should give an input box and a button (this works). But now I want to give an alert IF the input is hi and a different alert if the input is bye. For all other inputs there shouldn't happen anything. I'm just not getting it, can someone help out?

<h1>Welcome</h1>

    Input: <input id="welcome1" type="text" />

    <button id="button">Click here</button>
    
   </body>
   <script type="text/javascript">

    if (document.getElementById("welcome1").textContent = "hi"){
        document.getElementById("button").onclick = alert("Welcome");
    }
    else (document.getElementById("welcome1").textContent = "bye"){
        document.getElementById("button").onclick = alert("See you later");  
    }

   </script>