Typescript immutable array

immutableArray.ts

class ImmutableArray<T>  {

     private _data : Array<T>;

     public get value() : Array<T>{
         return this._data.slice(0);
     }

     constructor(data : Array<T>) {

         if(Array.isArray(data) !== true)
            throw new Error('data should represent an array');

         this._data = this.initialize(data);
     }

     private initialize(data : Array<T>) : Array<T> {

         return data.map(item => {

             var type = typeof (item);

             return type === 'object' ||
                    type === 'function' ? Object.freeze(item) : item;
         });
     }
 }