Javascript Prototype Gets in the Way!

I have been re-reading eloquent javascript because I am taking up a class this summer that greatly deals with javascript. This book really helped me a lot transitioning from programming solely in Java to understanding some of the practical details of using javascript.

The Prototype

One of the sections in the book that i found really helpful was the explanation of how prototypes work. Prototype Interference in this section of the book (Secret life of Objects). If you are beginning javascript, how the prototype works in javascript is one of the fundamental concepts that you must grasp. There are probably several prototypes in javascript but the three prototypes that you might want to know about are

There are a lot of things in each prototype above and I will not discuss each in detail. Ultimately, prototypes are the object that is used as a fallback if the property or method is not on the current object you are referencing. You can also add new properties and method to all objects based on a prototype. But be careful because it might cause problems and I have spent many hours trying to debug javascript because of issues caused by modifying the prototype.

An Issue in overriding prototype  

To save you time, here is the some the issue I have encountered with prototypes.

Let's say that you are part of a team using javascript. One of your teammates decided to add a property in the Object.prototype and you did not know this.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
//Let's mess with the objects property
//You did not know somebody added a property //in the objects prototype
Object.prototype.protoProperty = 'propertyInPrototype';
var map = {};
map['firstEntry'] ='firstEntry';
map['secondEntry'] ='secondEntry';

for(var entry in map){

 console.log(entry);
}


With the code example above let's say you did not know that line 3 happened. At line 4 you created a new object and proceeded to add your properties. Once you have all your properties set. You tried and loop thru your properties and voila a mystery property appears and you will wonder where is it coming from!


If the property really needs to be in the prototype, this can be solved in different ways.
You can do your loop this way in order to ensure that the property is owned by the object

1
2
3
4
5
6
for(var entry in map){
 //check if the map has this property
 if(map.hasOwnProperty(entry)){
 console.log(entry); 
 } 
}
Declare the property in the prototype to be non-enumerable 

 

1
Object.defineProperty (Object.prototype , "protoProperty" , { enumerable : false , value : " propertyInPrototype "}) ;

Or better yet create objects that are prototype less
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
//Prototype less object
var map2 = Object.create(null);
map2['firstEntry2'] ='firstEntry2';
map2['secondEntry2'] ='secondEntry2';
console.log('toString' in map2);
console.log('\n');
for(var entry in map2){

 console.log(entry);
}

Good Luck!

Adding properties and functions in prototypes is not such a bad thing but be aware of some of the side effects of doing so.

Comments

Post a Comment

Popular posts from this blog

OAuth 1.0a Request Signing and Verification - HMAC-SHA1 - HMAC-SHA256

Image
This is a security project that I did this Fall semester. I was interested in why the OAuth1.0a specification did not require you to sign a request using RSA-SHA1 or any RSA algorithm. The first issue that I found is where would you store your private keys in a javascript client? - as a javascript encrypted file ? from the browser ?  - that will be a security hole if you allow javascript to get private keys. These are some of the questions that got me asking and which I cannot find answers to for now. OAuth 1.0a Signature Signing and Verification using JavaScript Implementation Background The OAuth specification tries to define an API authentication method that allows consumer websites or applications to access protected API Resources from a Service provider. The advantage that OAuth provides is that the user of the website or application is not required to disclose their credentials to their service provider. Another advantage of using OAuth to authenticate is that it does not
Read more

Spark DataFrame - Array[ByteBuffer] - IllegalAurmentException

IllegalArgumentException - ByteBuffer - Spark DataFrame I was processing a several million documents (~ 20 million) in which we need to extract the NLP features using NLP4J, OpenNLP, and WordNet. The combination of the three NL features blows up each record to 11 times its original size. We are using all three because we do not know yet what feature sets will be helpful to us. The original dataset is in parquet files in HDFS (16 partitions). I thought that was convenient just use withColumn and pass a UDF (User Defined Function) on the column where it needs those features. withColumn adds the calculated column back to the DataFrame. So I created the spark job (I am on Spark 1.5.2-cdh5.5.2)for the above, and things started to get nasty. I am blowing up the ByteBuffer array on the in-memory columnar storage. This is the exception that I am getting. There seems to be no reference in my code in this stack trace. java.lang.IllegalArgumentException at java.nio.ByteB
Read more

Gensim Doc2Vec on Spark - a quest to get the right Vector

Ever since I joined the R & D group we have been doing a lot of cool things, like trying IBM Watson (see previous blog entry). Now we are doing a lot of Natural language processing.  We wanted to compare the similarity of two documents. There is this excellent project Gensim ( doc2vec ) that easily allow to you to translate large blocks of text to a fixed length feature vector to make comparisons. Here is the link to the  original paper  from some people from Google that explains the approach. In essence, they wanted to find a representation that will overcome the weaknesses of the bag of words model. The doc2vec approach proves to be a reliable approach to comparing similarities of documents because it takes into consideration the semantics and the order of the word in context. So with that, we wanted to use it for a corpus of 26 million documents. Calculating the doc2vec for 26 million documents is not a small task, so we need to process it in Spark. The problem is that there
Read more