"label1, label2, label3, label4, label5"

'label1','label2','label3','label4','label5'

var input = "label1, label2, label3, label4, label5";
var result = ''' + input.split(',').join('','') + ''';

"label1, label2, label3, label4, label5".split(',').map(function(word){
    return "'" + word.trim() + "'";
}).join(',');

"label1, label2, label3, label4, label5".split(',')
   .map(word => `'${word.trim()}'`)
   .join(',');

var str = "label1, label2, label3, label4, label5";
var arr = str.split(",");

for (var i in arr) {
    if (arr.hasOwnProperty(i)) {
        arr[i] = "'"+arr[i]+"'";
    }
}

var str = "label1, label2, label3, label4, label5";
var res = str.split(", ");
var data = "'" + res.join("','") + "'";
console.log(data);