To capitalize the first letter of a random string, you should follow these steps:

  1. Get the first letter of the string;
  2. Convert the first letter to uppercase;
  3. Get the remainder of the string;
  4. Concatenate the first letter capitalized with the remainder of the string and return the result;

1. Get the First Letter of the String

You should use the charAt() method, at index 0, to select the first character of the string.

var string = "freeCodecamp";

string.charAt(0); // Returns "f"

NOTE: charAt is preferable than using [ ] (bracket notation) as str.charAt(0) returns an empty string ('') for str = '' instead of undefined in case of ''[0].

2. Convert the First Letter to uppercase

You may use toUpperCase() method and convert the calling string to upper case.

var string = "freeCodecamp";

string.charAt(0).toUpperCase(); // Returns "F"

3. Get the Remainder of the String

You may use slice() method and get the remainder of the string (from the second character, index 1, to the end of the string).

var string = "freeCodecamp";

string.slice(1); // Returns "reeCodecamp"

4. Return the result adding the first letter and the remainder of the string

You should create a function that accepts a string as only argument and returns the concatenation of the first letter capitalized string.charAt(0).toUpperCase() and the remainder of the string string.slice(1).

var string = "freeCodecamp";

function capitalizeFirstLetter(str) {
  return str.charAt(0).toUpperCase() + str.slice(1);

capitalizeFirstLetter(string); // Returns "FreeCodecamp"

Or you may add that function to the String.prototype for using it directly on a string using the following code (so that the method is not enumerable but can be overwritten or deleted later):

var string = "freeCodecamp";

/* this is how methods are defined in prototype of any built-in Object */
Object.defineProperty(String.prototype, 'capitalizeFirstLetter', {
    value: function () {
        return this.charAt(0).toUpperCase() + this.slice(1);
    writable: true, // so that one can overwrite it later
    configurable: true // so that it can be deleted later

string.capitalizeFirstLetter(); // Returns "FreeCodecamp"


stackoverflow - Capitalize the first letter of string in JavaScript