amountOfCertainCharacter

String.prototype.amountOfCertainCharacter = function(char) {
    return this.split("").filter(k => k === char).length
}

// example
// code | expected output

"testing".amountOfCertainCharacter("t") // 2
"testing".amountOfCertainCharacter("e") // 1

"testing".amountOfCertainCharacter()    // 0
"testing".amountOfCertainCharacter("q") // 0