JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.indexOfUndefined =  function() {
    for ( var i = 0; i < this.length; i++ ) {
        if ( typeof this[i] === "undefined" ) {
            return i;
        }
    }
}
a=[1,2,3,undefined,4,5];
alert("The index of undefinedis: " + a.indexOfUndefined());