TypedObject Constructor

proxy-typed-object.js

(function( global ) {

  global.TypedObject = function( obj ) {

    var keys = Object.keys( obj || {} ),
        len = keys.length,
        data = {},
        typeOf = {
          // prop: type
        },
        k = -1;

    if ( len ) {
      while ( ++k < len ) {
        var key = keys[ k ], 
            val = obj[ key ];
        data[ key ] = val;
        typeOf[ key ] = typeof val;
      }
    }

    return Proxy.create({
      get: function( r, name ) {
        return data;
      },
      set: function( r, name, value ) {

        var type = typeOf[ name ];

        if ( !type ) {
          type = typeOf[ name ] = typeof value;
        }

        if ( typeof value !== type ) {
          throw new Error(
            "Property value type mismatch for `" + name + "`: expected " + type + ", saw " + typeof value
          );
        }

        data[ name ] = value;
      }
    }, Object.prototype );
  };

})( this );

var o = new TypedObject();

o.foo = "string";

try {
 o.foo = 1; 
} catch(e) {
  console.log( e );  
  // > "Property value type mismatch for `foo`: expected string, saw number
}

console.log( o );

o = new TypedObject({ foo: 1 });

console.log( o );

try {
 o.foo = []; 
} catch(e) {
  console.log( e );  
  // > "Property value type mismatch for `foo`: expected number, saw object
}

http://jsbin.com/esacik/10