You can check if a string is a valid number using isNaN. So

if(isNaN(value)) {
    // value is email
 } else {
    // value is id
 }

Your code sample is incomplete but what you're trying to do should look something like this. Rather than assuming any non-numerical response is a valid email, which can break in the future as the API changes, be explicit about what you're looking for as below.

// Assume you're comfortable using a relatively loose pattern to match an email i.e. no overly strict formatting rules
const mailPattern = /^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/
// Assume response is in scope and contains the API response with the email or id deserialized from the body
const value = mailPattern.test(response) ? value : null

You can use Regular expression /^\d+$/ combined with RegExp.prototype.test():

if (/^\d+$/.test(value)) {
  // It's number
}