https://en.wikipedia.org/wiki/Weasel_program

weasel program.js

var alphabet = "ABCDEFGHIJKLMONPQRSTUVWXYZ ",
    target = "METHINKS IT IS LIKE A WEASEL",
    i, score = 0, copies, gen = 0, child = "";

String.prototype.replaceAt = function(index, character) {
    return this.substr(0, index) + character + this.substr(index+1);
}

String.prototype.shuffle = function() {
    var i = this.length, j, temp = this;
    while ( --i ) {
        j = generateRandom(i + 1);
        temp = temp.replaceAt(i, temp[j]);
    }
    return temp;
}

String.prototype.makeMutations = function() {
    var i, newChar, str = this;

    for(i=0; i<str.length; i++){
        if( Math.floor(Math.random() * 100) < 5 ){
			newChar = alphabet[generateRandom(alphabet.length)];
            str = str.replaceAt(i, newChar);
        }
    }

    return str;
}

String.prototype.calculateScore = function() {
    var i, score=0, str = this;

    for(i=0; i<str.length; i++){
        if(str[i] == target[i]){
            score++;
        }
    }

    return score;
}

function generateRandom(max){
    return Math.floor(Math.random() * max)
}

//Start with a random string of 28 characters.
child = alphabet.shuffle()

copies = [];
for(i=0; i<100; i++){
    //For each character in each of the 100 copies, with a probability of 5%, replace (mutate) the character with a new random character.
    child = child.makeMutations();

    //Make 100 copies of the string (reproduce).
	copies.push(child);

    //Compare each new string with the target string "METHINKS IT IS LIKE A WEASEL", and give each a score (the number of letters in the string that are correct and in the correct position).
    console.log(i + ": " + child + " -- score: " + child.calculateScore());
}