JSFiddle - React, Tailwind, and code Playground

JavaScript

var correct = "Honolulu";
var input1  = "Hnolulu";
var input2  = "Hoblulu";
var input3  = "Honoluluss";

console.log( validate(correct, input1) ); // 1
console.log( validate(correct, input2) ); // 2
console.log( validate(correct, input3) ); // 8

function validate(correct, input) {
    var error = -1;
    var comp1 = correct.length > input.length ? correct : input;
    var comp2 = correct.length > input.length ? input : correct;
    
    for (var i=0; i<comp1.length; i++) {
        if (comp1.charAt(i) != comp2.charAt(i)) {
            error = i;
            break;
        }
    }
    return error;
}