Edit in JSFiddle

/**
 * Creates a "language" where:
 * a results in printing "Hello "
 * b results in printing "World!\n"
 * anything else is ignored
 */
var myLanguage = function (input) {
    var logStr = '';
    input = input.split('');
    input.forEach(function (item) {
        if (item === 'a') {
            logStr += 'Hello ';
        } else if (item === 'b') {
            logStr += 'World!\n';
        }
    });
    console.log(logStr);
};

myLanguage('abc');