Uses a slightly modified version of the Levenshtien Distance algorithm (also known as "edit distance") to calculate the set of the fewest operations which will change string s in to string t.

gistfile2.txt

# Output of the example which is console.logged at the bottom of operations.js:

$ node operations.js 
To change 'kitten' in to 'sitting':
[ [ 'delete', 'k' ]
, [ 'insert', 's' ]
, [ 'retain', 1 ]
, [ 'retain', 1 ]
, [ 'retain', 1 ]
, [ 'delete', 'e' ]
, [ 'insert', 'i' ]
, [ 'retain', 1 ]
, [ 'insert', 'g' ]
]

operations.js

(function () {

    // Simple operation constructors.

    function insert (chars) {
        return ["insert", chars];
    }

    function del (chars) {
        return ["delete", chars];
    }

    function retain (n) {
        return ["retain", n];
    }

    // We don't want to copy arrays all the time, aren't mutating lists, and
    // only need O(1) prepend and length, we can get away with a custom singly
    // linked list implementation.

    var theEmptyList = {
        length: 0,
        toArray: function () {
            return [];
        }
    };

    function toArray () {
        return [this.car].concat(this.cdr.toArray());
    }

    function cons (car, cdr) {
        return {
            car: car,
            cdr: cdr,
            length: 1 + cdr.length,
            toArray: toArray
        };
    }

    // Abstract out the table in case I want to change the implementation to
    // arrays of arrays or something.

    function put (table, x, y, ops) {
        return (table[String(x) + "," + String(y)] = ops);
    }

    function get (table, x, y) {
        var ops = table[String(x) + "," + String(y)];
        if ( ops ) {
            return ops;
        } else {
            throw new TypeError("No operations at " + String(x) + "," + String(y));
        }
    }

    function makeOperationsTable (s, t) {
        var table = {},
            n = s.length,
            m = t.length,
            i,
            j;
        put(table, 0, 0, theEmptyList);
        for ( i = 1; i <= m; i += 1 ) {
            put(table, i, 0, cons(insert(t.charAt(i-1)),
                                  get(table, i-1, 0)));
        }
        for ( j = 1; j <= n; j += 1 ) {
            put(table, 0, j, cons(del(s.charAt(j-1)),
                                  get(table, 0, j-1)));
        }
        return table;
    }

    function chooseCell (table, x, y, k) {
        var prevOps = get(table, x, y-1),
            min = prevOps.length,
            direction = "up";

        if ( get(table, x-1, y).length < min ) {
            prevOps = get(table, x-1, y);
            min = prevOps.length;
            direction = "left";
        }

        if ( get(table, x-1, y-1).length < min ) {
            prevOps = get(table, x-1, y-1);
            min = prevOps.length;
            direction = "diagonal";
        }

        return k(direction, prevOps);
    }

    function operations (s, t) {
        var n = s.length,
            m = t.length,
            i,
            j,
            ops = makeOperationsTable(s, t);
        for ( i = 1; i <= m; i += 1 ) {
            for ( j = 1; j <= n; j += 1 ) {
                chooseCell(ops, i, j, function (direction, prevOps) {
                    switch ( direction ) {
                    case "left":
                        put(ops, i, j, cons(insert(t.charAt(i-1)), prevOps));
                        break;
                    case "up":
                        put(ops, i, j, cons(del(s.charAt(j-1)), prevOps));
                        break;
                    case "diagonal":
                        if ( s.charAt(j-1) === t.charAt(i-1) ) {
                            put(ops, i, j, cons(retain(1), prevOps));
                        } else {
                            put(ops, i, j, cons(insert(t.charAt(i-1)),
                                                cons(del(s.charAt(j-1)),
                                                     prevOps)));
                        }
                        break;
                    default:
                        throw new TypeError("Unknown direction.");
                    }
                });
            }
        }
        return get(ops, i-1, j-1).toArray().reverse();
    }

    exports = typeof exports !== "undefined"
        ? exports
        : this;

    exports.operations = operations;

    console.log("To change 'kitten' in to 'sitting':");
    console.log(operations("kitten", "sitting"));

}());

And if Lisp is your thing: https://github.com/vsedach/vas-string-metrics