function spinalCase(str) {
    var re = /[a-z][A-Z]|\_|\ /g;
   // str = str[0].toLowerCase() + str.slice(1);

    str = str.replace(re, (match)=>{
        switch(match){
            case "_":
            case " ":
                return "-";
            default:
                [a,b] = match.split("");
                return a + "-" + b;
        }
    });
    return str.toLowerCase();
}

spinalCase('This Is Spinal Tap');