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JavaScript Node.js Basics 2017 Building a Command Line Application Parsing JSON

Brian Patterson
Brian Patterson
19,588 Points
Posted by Brian Patterson
Brian Patterson
19,588 Points

TypeError: Cannot read property 'length' of undefined

I am trying to get some practice with using Node.js . The problem I am having is parsing the data as I am getting the error above. Below is my code.

const http = require('http');

const season = 1971;

function printMessage(races, season) {
  const message = `There are ${season}  rounds at the ${races}.`;
  console.log(message);
}

//printMessage(22,2017);
//Connecting to the API url (http://ergast.com/api/f1/2012.json)
const request = http.get(`http://ergast.com/api/f1/${season}.json`,response => {
  //console.dir(response.statusCode);
    let body = "";
    //Read the data
  response.on('data', data => {
    //Changing the buffer into a string.
    body += data;
  });
  response.on('end', () => {
    //Parse the data
    const seasonF1 = JSON.parse(body);
    // console.log(body);
    // console.log(typeof body);
    //console.dir(seasonF1);
    printMessage(season, seasonF1.Races.length);
    //Print the data
  });

});

Below is the error I am getting in the console.

/Users/briankaty1/Dropbox/JavaScript/testApp/app.js:26
    printMessage(season, seasonF1.Races.length);
                                       ^

TypeError: Cannot read property 'length' of undefined
    at IncomingMessage.response.on (/Users/briankaty1/Dropbox/JavaScript/testApp/app.js:26:40)
    at emitNone (events.js:91:20)
    at IncomingMessage.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:974:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

Any help would be appreciated.

2 Answers

andren
andren
28,558 Points
andren
andren
28,558 Points

The issue is that there is no property named "Races" at the root of the JSON that you get from that URL.

Here is the parsed JSON:

{
  MRData: {
    xmlns: "http://ergast.com/mrd/1.4",
    series: "f1",
    url: "http://ergast.com/api/f1/1971.json",
    limit: "30",
    offset: "0",
    total: "11",
    RaceTable: {
      season: "1971",
      Races: [...] 
    }
  }
}

As you can see the Races array is found within an object called RaceTable, which itself is found within MRData.

If you change your print statement to reflect that structure like this:

printMessage(season, seasonF1.MRData.RaceTable.Races.length);

Then your code should work.

EDIT: Added a view of the JSON you receive.

Tuan Phan
Tuan Phan
10,825 Points
Tuan Phan
Tuan Phan
10,825 Points

I used [] instead of . to access the value in array.

Works well for me.

My case:

printMessage(profile['name'], profile['badges'].length, profile['points']['JavaScript']);

Yours maybe work with this

    printMessage( season, seasonF1['Races'].length );