<input type="number" name="cp" value="cp" onchange="getCP(this.value)">   

function getCP(cp) {
    var cp = cp;
    const getVille = async function(){
        try{
            let response = await fetch('localhost/projet/pages/cp.php.cp='cp'')
            if(response.ok){
                let data = await response.json()
                console.log(data)
            }else{
                console.error('retour du serveur:'response.status)
            }
        }catch(e){
            console.log(e)
        }
    }
getVille();
}

Uncaught ReferenceError: getCP is not defined
    at HTMLInputElement.onchange

function getCP(cp) {
console.log(cp);

alerte (cp)
}