static/ext/src/util/AbstractMixedCollection.js
function(o) {
        var me = this,
            key;

        if (o != null) {
            // If
            //     We have not been forced into using linear lookup by a usage of the 2 arg form of add
            // and
            //     The key extraction function yields a key
            // Then use the map to determine object presence.
            if (!me.useLinearSearch && (key = me.getKey(o))) {
                return this.map[key] != null;
            }

            // Fallback: Use linear search
            return Ext.Array.indexOf(this.items, o) !== -1;
        }
        
        return false;
    }
function(o) {
        var me = this,
            key;

        if (o != null) {
            // If
            //     We have not been forced into using linear lookup by a usage of the 2 arg form of add
            // and
            //     The key extraction function yields a key
            // Then use the map to determine object presence.
            if (!me.useLinearSearch && (key = me.getKey(o))) {
                return this.map[key] != null;
            }

            // Fallback: Use linear search
            return Ext.Array.indexOf(this.items, o) !== -1;
        }
        
        return false;
    }
Returns true if the collection contains the passed Object as an item.
@param {Object} o  The Object to look for in the collection.
@return {Boolean} True if the collection contains the Object as an item.
@since Ext 1