JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <form onsubmit="return false" method="POST" action"/" >
        <input type="text" id="nom" placeholder="Nombre" />
        <input type="password" id="pass" placeholder="Contraseña" onfocusout="chkpass()" />
        <input type="submit" value="Enviar" />
    </form>
</body>

JavaScript

function chkpass(){
    if(true){ //Aquí haces tus comprobaciones
        alert("El password debe ser al menos de 8 caracteres.")
                document.getElementById("pass").value=("");
                document.getElementById("pass").focus();
        
    }
}