Axios response data is undefined while the request is succesful

The code you have posted isn't synchronous the alert will trigger before your http request has a chance to resolve which is probably why you get an undefined. if you want to do anything with the response body you should probably do it within the 'then' callback.

function confirmPassword() {
    let password = document.getElementById("password").value
    let email = document.getElementById("email").value

// 1.your service call through axios gets triggered
    Service.confirmPassword(email, password).then(response => {
      // 3.this gets execute after the alert below once the http request has been resolved
        res = response.data 
        setTwoFactorStep(1);
    // invoke your handler here...
    }).catch(err => {
        alert(err.data.message)
    })
    alert(res) // 2.this gets executed next
}  

if you want to write this in a synchronous way, consider using async/await

 async function confirmPassword() {
    let password = document.getElementById("password").value
    let email = document.getElementById("email").value

    const response = await Service.confirmPassword(email, password).catch(err => alert(err.data.message));
    res = response.data;
    setTwoFactorStep(1);
    alert(res)