How does this replaceAt function work?

Question

Could you please explain how this piece of code works?

String.prototype.replaceAt = function(index, character) {
    return this.substr(0, index) + characte         


        

Answer 1

The replaceAt function simply takes the index of a character (0 in this case) and replaces it with another character (in this case the uppercase version of the original character. This specific function is just Title Casing a word by replacing the first character with the same character in uppercase.

The line that your questioning, takes a substring of the word before the character at the specificied index this.substr(0,index) since substr is non-inclusive of the last index, appends the specified character + character, and appends a substr of the rest of the word + this.substr(index+character.length)

Example 'testing'.replaceAt(0,testing.charAt(0).toUpperCase()); = '' + 'T' + 'esting' = Testing;