Simplistic Javascript Observer - Novice

observer.js

var T_HTML = 'T_HTML';
var T_VAL = 'T_VAL';
var T_TEXT = 'T_TEXT';
var T_FUNCTION = 'T_FUNCTION';

window.Observer = {
    types: [T_HTML, T_VAL, T_TEXT, T_FUNCTION],
    add: function (key, value, observer, type) {
        var that = this;

        if (that.types.indexOf(type) < 0) {
            console.log(' ! Observer of type(' + type + ') not supported');
            return;
        }

        if (that[key] === undefined) {
            that[key] = {
                value: value,
                observers: [{
                    observer: observer,
                    type: type
                }]
            };
            return;
        }

        //TODO: Check for redundant entries

        that[key].observers.push({observer: observer, type: type});
    },

    first: function (key, value, observer, type) {
        this.remove(key);
        this.add(key, value, observer, type);
    },

    change: function (key, value) {

        if (this[key] === undefined) {
            return;
        }

        this[key].value = value;
        this.observe(this[key]);
    },

    observe: function (v) {
        var that = this;
        v.observers.forEach(function (observer) {
            that.observeValue(observer, v.value);
        })
    },

    observeValue: function (observer, value) {
        switch (observer.type) {
            case T_HTML:
                $$(observer.observer).html(value || '');
                break;

            case T_VAL:
                $$(observer.observer).val(value);
                break;

            case T_TEXT:
                $$(observer.observer).text(value);
                break;

            case T_FUNCTION:
                observer.observer();
                break;

        }
    },

    remove: function (key) {
        delete this[key];
    }

};

// How to attach observer
Observer.first('var_name', null, '#my-first-selector', T_TEXT);

// Trigger Change
Observer.change('var_name', 'rummykhan');

// Add another observer
Observer.add('var_name', 'rummykhan', '.my-second-selector', T_TEXT);

// Trigger Change
Observer.change('var_name', 'jumangi');