js memento(undo redo)

memento.js

function Memento(originalState) {
    this.state = originalState;
    this.stack = new Array(this.state);
    this.stackIndex = 0;
}
Memento.prototype.undo = function () {
    return this.stackIndex > 0 ? 
        this.state = this.stack[--this.stackIndex]: this.state;
};
Memento.prototype.save = function (state) {
    this.stack.length = ++this.stackIndex + 1;
    return this.state = this.stack[this.stackIndex] = state;
};
Memento.prototype.redo = function () {
    return this.stackIndex < this.stack.length - 1 ? 
        this.state = this.stack[++this.stackIndex] : this.state;
};