Issue with TypeError using a npm hashtable in a object declaration

Question

I'm trying to get a simple stream working to read and parse JSON and add pairs to a hashtable. Eventually I am going to export this as a module to use in another program, but in debugging I'm stuck at this error:

 this.hashtable.put(data['word'], data['rate']);
                     ^

TypeError: Cannot read property 'put' of undefined
    at Stream.<anonymous> (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/test/test-stream.js:18:22)
    at Stream.emit (events.js:315:20)
    at drain (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/through/index.js:36:16)
    at Stream.stream.queue.stream.push (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/through/index.js:45:5)
    at Parser.parser.onValue (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/JSONStream/index.js:118:16)
    at Parser.proto.emit (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/jsonparse/jsonparse.js:337:8)
    at Parser.proto.pop (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/jsonparse/jsonparse.js:332:8)
    at Parser.proto.onToken (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/jsonparse/jsonparse.js:402:12)
    at Parser.parser.onToken (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/JSONStream/index.js:128:12)
    at Parser.proto.write (/mnt/c/users/joeys/Desktop/RumbleUp/message-data/dev/node_modules/jsonparse/jsonparse.js:135:34)

Here is my simple script:

var JSONStream = require('JSONStream')
var fs = require('fs');
var Hash = require('simple-hashtable');


function obj() {
  this.hashtable = new Hash();

  console.log(this.hashtable.isEmpty());
  this.hashtable.put('yo', 'response');


  this.stream = fs.createReadStream('./output-1.json', { encoding: 'utf8' }).pipe(JSONStream.parse('*'));

  this.read = function(){
    this.stream.on('data', function(data){
        console.log(data['word']);
        this.hashtable.put(data['word'], data['rate']);
        //console.log(this.hashtable.get(data['word']));

    });
  }
}

var temp = new obj();
temp.read();

The JSON file is usually very large, hence the stream, but in this testing it is simply this:

[{
  "word": "Co",
  "rate": 0.20654307524536533
},{
  "word": "Issac",
  "rate": 0.08174386920980926
},{
  "word": "httpsaswarus",
  "rate": 0.12835820895522387
},{
  "word": "values",
  "rate": 0.2151509086923788
}]

When I debug, the hashtable is constructed fine and isn't undefined. This methods outside of the read function both work fine. When I remove the line causing the error, everything prints perfectly fine. I just don't understand why the table would be undefined if it works fine in those two methods. I feel like I'm missing something. If anyone has any insight on solving this issue, it would be much appreciated!

Here are links to the packages I used for reference:

https://www.npmjs.com/package/simple-hashtable

https://www.npmjs.com/package/JSONStream

Does this answer your question? [How does the "this" keyword work?](https://stackoverflow.com/questions/3127429/how-does-the-this-keyword-work) — Robin Zigmond, Jul 23 '20 at 21:23

score 0 · Accepted Answer · answered Jul 23 '20 at 21:20

I believe you are running into this issue because this has a different meaning within your callback function.

this.stream.on('data', function(data){
   console.log(data['word']);
   this.hashtable.put(data['word'], data['rate']); // this isn't this
   //console.log(this.hashtable.get(data['word']));
});

You could resolve this by doing something as simple as:

const that = this;
this.stream.on('data', function(data){
   console.log(data['word']);
   that.hashtable.put(data['word'], data['rate']); // that, not this
   //console.log(this.hashtable.get(data['word']));
});

Or, you could change the meaning of this within the callback function by doing:

this.stream.on('data', function(data){
   console.log(data['word']);
   this.hashtable.put(data['word'], data['rate']); // this is this
   //console.log(this.hashtable.get(data['word']));
}.bind(this));

I would also mention the modern ES6 solution of making the callback an arrow function instead — Robin Zigmond, Jul 23 '20 at 21:22

Issue with TypeError using a npm hashtable in a object declaration

1 Answers1