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Parsing JSON

Since many services use JSON for exchanging information, wouldn't it be nice to know how to do this in Free Pascal and Lazarus?

How do I make a GET request?

Note

The snippet below is based on a fine tutorial by Marcus Fernström.

Watch the detailed explanation by Marcus on YouTube; Learn how to consume JSON data in Free Pascal

Here is an example on making a GET request of JSON data and displaying it on console.

  1. In uses add opensslsockets and fphttpclient for making the GET request. Line 11-12.
  2. Set two variables. Line 16-17.
    • One for the URL. Here I use https://dummyjson.com/users?limit=3.
    • One for the response of the GET request.
  3. Make the GET request by using TFPHTTPClient.SimpleGet(url). Line 22.
  4. Print result on console. Line 25.
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program GetRequest;

{$mode objfpc}{$H+}{$J-}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  Classes,
  // For making web requests
  opensslsockets,
  fphttpclient;


var
  url: string = 'https://dummyjson.com/users?limit=3'; // endpoint to get JSON mock data
  rawJson: string; // a var to store raw JSON data

begin
  // Get the raw JSON data
  WriteLn('Contacting ', url, ' ...');
  rawJson := TFPHTTPClient.SimpleGet(url);

  // Display JSON on console
  WriteLn(rawJson);

  // Pause console
  WriteLn('Press enter key to exit...');
  ReadLn;
end.

How do I parse JSON from dummyjson.org?

Note

Again, the snippet below is based on a fine tutorial by Marcus Fernström.

Watch the detailed explanation by Marcus on YouTube; Learn how to consume JSON data in Free Pascal

Here is the breakdown of the example below.

  1. Add necessary units for making the GET requests and parsing JSON data. Line 10-15.
  2. Setup variables for receiving and processing the JSON data. Line 19-22.
    • A string to store raw JSON data.
    • A TJSONArray for storing an array of JSON objects.
    • A TJSONEnum for looping JSON arrays.
    • A TJSONObject for manipulating a JSON object.
  3. Make the GET request and store the raw JSON response. Line 27.
  4. Retrieve the array as TJSONArray. Line 33.
    • Convert the raw JSON data to TJSONData,
    • find data called "users" (a JSON array as per dummyjson's specs) and
    • Lastly, cast as TJSONArray.
  5. Use the TJSONEnum var to loop through the JSON array. Line 36.
  6. To use the JSON object in the loop, cast the TJSONEnum .value, which is TJSONData to TJSONObject. Line 39.
  7. Once you've obtained the JSONObject, use FindPath to get value based on a key or a path.
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program ParseJSON;

{$mode objfpc}{$H+}{$J-}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  Classes,
  // For web requests
  opensslsockets,
  fphttpclient,
  // For parsing JSON data
  fpjson,
  jsonparser;

var
  url: string = 'https://dummyjson.com/users?limit=3'; // endpoint to get JSON mock data
  rawJson: string;       // a var to store the raw JSON data
  arrayJson: TJSONArray; // a var for storing an array of JSON objects
  enumJson: TJSONEnum;   // an enum type for looping JSON arrays
  objJson: TJSONObject;  // a var for manipulating a JSON object

begin
  // Get the raw JSON data
  WriteLn('Contacting ', url, ' ...');
  rawJson := TFPHTTPClient.SimpleGet(url);

  // Next, get the users array as TJSONArray;
  // 1. convert the raw JSON data to TJSONData,
  // 2. find data called "users" (a JSON array as per dummyjson's structure) and
  // 3. cast as TJSONArray
  arrayJson := TJSONArray(GetJSON(rawJson).FindPath('users'));

  // Loop using the TJSONEnum
  for enumJson in arrayJson do
  begin
    // Cast the enum value (TJSONData) to TJSONObject
    objJson := TJSONObject(enumJson.Value);

    // Output values based on keys/paths using `TJSONObject.FindPath('a_key')`
    WriteLn('id   : ', objJson.FindPath('id').AsString);
    WriteLn('name : ', objJson.FindPath('firstName').AsString, ' ', objJson.FindPath('lastName').AsString);
    WriteLn('phone: ', objJson.FindPath('phone').AsString);
    WriteLn('city : ', objJson.FindPath('address.city').AsString);
    WriteLn('state: ', objJson.FindPath('address.state').AsString);
    WriteLn('---');
  end;

  // Pause console
  WriteLn('Press enter key to exit...');
  ReadLn;
end.

Why do we use GetJSON(rawJson).FindPath('users') to get the array?

As per the specs of https://dummyjson.com/users?limit=3, the array of users is in users. Here is an response example from the /users?limit=3 endpoint.

{
  "users": [
    {
      "id": 1,
      "firstName": "Terry",
      "lastName": "Medhurst",
      "maidenName": "Smitham",
      "age": 50,
      "gender": "male",
      "email": "atuny0@sohu.com",
      "phone": "+63 791 675 8914",
      "username": "atuny0",
      "password": "9uQFF1Lh",
      "birthDate": "2000-12-25",
      "image": "https://robohash.org/Terry.png?set=set4",
      "bloodGroup": "A-",
      "height": 189,
      "weight": 75.4,
      "eyeColor": "Green",
      "hair": {
        "color": "Black",
        "type": "Strands"
      },
      "domain": "slashdot.org",
      "ip": "117.29.86.254",
      "address": {
        "address": "1745 T Street Southeast",
        "city": "Washington",
        "coordinates": {
          "lat": 38.867033,
          "lng": -76.979235
        },
        "postalCode": "20020",
        "state": "DC"
      },
      "macAddress": "13:69:BA:56:A3:74",
      "university": "Capitol University",
      "bank": {
        "cardExpire": "06/22",
        "cardNumber": "50380955204220685",
        "cardType": "maestro",
        "currency": "Peso",
        "iban": "NO17 0695 2754 967"
      },
      "company": {
        "address": {
          "address": "629 Debbie Drive",
          "city": "Nashville",
          "coordinates": {
            "lat": 36.208114,
            "lng": -86.58621199999999
          },
          "postalCode": "37076",
          "state": "TN"
        },
        "department": "Marketing",
        "name": "Blanda-O'Keefe",
        "title": "Help Desk Operator"
      },
      "ein": "20-9487066",
      "ssn": "661-64-2976",
      "userAgent": "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/534.24 (KHTML, like Gecko) Chrome/12.0.702.0 Safari/534.24",
      "crypto": {
        "coin": "Bitcoin",
        "wallet": "0xb9fc2fe63b2a6c003f1c324c3bfa53259162181a",
        "network": "Ethereum (ERC20)"
      }
    },
    {
      "id": 2,
      "firstName": "Sheldon",
      "lastName": "Quigley",
      "maidenName": "Cole",
      "age": 28,
      "gender": "male",
      "email": "hbingley1@plala.or.jp",
      "phone": "+7 813 117 7139",
      "username": "hbingley1",
      "password": "CQutx25i8r",
      "birthDate": "2003-08-02",
      "image": "https://robohash.org/Sheldon.png?set=set4",
      "bloodGroup": "O+",
      "height": 187,
      "weight": 74,
      "eyeColor": "Brown",
      "hair": {
        "color": "Blond",
        "type": "Curly"
      },
      "domain": "51.la",
      "ip": "253.240.20.181",
      "address": {
        "address": "6007 Applegate Lane",
        "city": "Louisville",
        "coordinates": {
          "lat": 38.1343013,
          "lng": -85.6498512
        },
        "postalCode": "40219",
        "state": "KY"
      },
      "macAddress": "13:F1:00:DA:A4:12",
      "university": "Stavropol State Technical University",
      "bank": {
        "cardExpire": "10/23",
        "cardNumber": "5355920631952404",
        "cardType": "mastercard",
        "currency": "Ruble",
        "iban": "MD63 L6YC 8YH4 QVQB XHIK MTML"
      },
      "company": {
        "address": {
          "address": "8821 West Myrtle Avenue",
          "city": "Glendale",
          "coordinates": {
            "lat": 33.5404296,
            "lng": -112.2488391
          },
          "postalCode": "85305",
          "state": "AZ"
        },
        "department": "Services",
        "name": "Aufderhar-Cronin",
        "title": "Senior Cost Accountant"
      },
      "ein": "52-5262907",
      "ssn": "447-08-9217",
      "userAgent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.30 (KHTML, like Gecko) Ubuntu/11.04 Chromium/12.0.742.112 Chrome/12.0.742.112 Safari/534.30",
      "crypto": {
        "coin": "Bitcoin",
        "wallet": "0xb9fc2fe63b2a6c003f1c324c3bfa53259162181a",
        "network": "Ethereum (ERC20)"
      }
    },
    {
      "id": 3,
      "firstName": "Terrill",
      "lastName": "Hills",
      "maidenName": "Hoeger",
      "age": 38,
      "gender": "male",
      "email": "rshawe2@51.la",
      "phone": "+63 739 292 7942",
      "username": "rshawe2",
      "password": "OWsTbMUgFc",
      "birthDate": "1992-12-30",
      "image": "https://robohash.org/Terrill.png?set=set4",
      "bloodGroup": "A-",
      "height": 200,
      "weight": 105.3,
      "eyeColor": "Gray",
      "hair": {
        "color": "Blond",
        "type": "Very curly"
      },
      "domain": "earthlink.net",
      "ip": "205.226.160.3",
      "address": {
        "address": "560 Penstock Drive",
        "city": "Grass Valley",
        "coordinates": {
          "lat": 39.213076,
          "lng": -121.077583
        },
        "postalCode": "95945",
        "state": "CA"
      },
      "macAddress": "F2:88:58:64:F7:76",
      "university": "University of Cagayan Valley",
      "bank": {
        "cardExpire": "10/23",
        "cardNumber": "3586082982526703",
        "cardType": "jcb",
        "currency": "Peso",
        "iban": "AT24 1095 9625 1434 9703"
      },
      "company": {
        "address": {
          "address": "18 Densmore Drive",
          "city": "Essex",
          "coordinates": {
            "lat": 44.492953,
            "lng": -73.101883
          },
          "postalCode": "05452",
          "state": "VT"
        },
        "department": "Marketing",
        "name": "Lindgren LLC",
        "title": "Mechanical Systems Engineer"
      },
      "ein": "48-3951994",
      "ssn": "633-89-1926",
      "userAgent": "Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:21.0.0) Gecko/20121011 Firefox/21.0.0",
      "crypto": {
        "coin": "Bitcoin",
        "wallet": "0xb9fc4b4b855bc44eb30d5e36fd18f491f44a15b7",
        "network": "Ethereum (ERC20)"
      }
    }
  ],
  "total": 100,
  "skip": 0,
  "limit": 3
}