toCapitalize.js

String.prototype.toCapitalize = function () {
    let arr = this.toLowerCase().split(" ");

    arr.forEach((item, index) => {
        arr[index] = item[0].toUpperCase() + item.substr(1);
    });

    return arr.join(" ");
}

let string = `hello world! this is TEST string`;
console.log(string.toCapitalize());