JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="champdunom" id="un"/>
  <input type="submit" name="submit"/>

JavaScript

var valeurchamp = document.getElementById('un').value; 

function envoyer(num){
alert('Test');   
}

document.getElementById('un').addEventListener("blur", function(){
    envoyer(1);
}, false);