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JavaScript Object Notation (JSON)은 Javascript 객체 문법으로 구조화된 데이터를 표현하기 위한 문자 기반의 표준 포맷이다. 웹 어플리케이션에서 데이터를 전송할 때 일반적으로 사용한다(서버에서 클라이언트로 데이터를 전송하여 표현하려거나 반대의 경우). 여기저기서 자주 보았을테니 여기선 JSON을 파싱, 데이터에 접근하고 JSON을 생성하는 등 Javascript로 JSON을 다루는 법에 대해 알아봅시다.

사전지식: 컴퓨터 기초, HTML과 CSS에 대한 기본 지식, Javascript 기초 (First stepsBuilding blocks 참조), OOJS 기초 (Introduction to objects 참조).
목표: JSON에 담긴 데이터를 이용하는 법과 JSON 객체 생성하는 법에 대해 알아보기.

아니, 대체 JSON이 뭐죠?

JSONDouglas Crockford가 널리 퍼뜨린 Javascript 객체 문법을 따르는 문자 기반의 데이터 포맷입니다. JSON이 Javascript 객체 문법과 매우 유사하지만 딱히 Javascript가 아니더라도  JSON을 읽고 쓸 수 있는 기능이 다수의 프로그래밍 환경에서 제공됩니다.

JSON은 문자열 형태로 존재합니다 — 네트워크를 통해 전송할 때 아주 유용하죠. 데이터에 억세스하기 위해서는 네이티브 JSON 객체로 변환될 필요가 있습니다. 별로 큰 문제는 아닌 것이 Javascript는 JSON 전역 객체를 통해 문자열과 JSON 객체의 상호변환을 지원합니다.

Note: 문자열에서 네이티브 객체로 변환하는 것은 파싱(Parsing)이라고 합니다. 네트워크를 통해 전달할 수 있게 객체를 문자열로 변환하는 과정은 문자열화(Stringification)이라고 합니다.

개별 JSON 객체를 .json 확장자를 가진 단순 텍스트 파일에 저장할 수 있습니다. MIME 타입application/json 입니다.

JSON 구조

위에서 설명했듯이 JSON은 Javascript 객체 리터럴 문법을 따르는 문자열입니다. JSON 안에는 마찬가지로 Javascript의 기본 데이터 타입인 문자열, 숫자, 배열, 불리언 그리고 다른 객체를 포함할 수 있습니다. 이런 방식으로 여러분은 데이터 계층을 구축할 수 있습니다, 아래 처럼요.

{
  "squadName": "Super hero squad",
  "homeTown": "Metro City",
  "formed": 2016,
  "secretBase": "Super tower",
  "active": true,
  "members": [
    {
      "name": "Molecule Man",
      "age": 29,
      "secretIdentity": "Dan Jukes",
      "powers": [
        "Radiation resistance",
        "Turning tiny",
        "Radiation blast"
      ]
    },
    {
      "name": "Madame Uppercut",
      "age": 39,
      "secretIdentity": "Jane Wilson",
      "powers": [
        "Million tonne punch",
        "Damage resistance",
        "Superhuman reflexes"
      ]
    },
    {
      "name": "Eternal Flame",
      "age": 1000000,
      "secretIdentity": "Unknown",
      "powers": [
        "Immortality",
        "Heat Immunity",
        "Inferno",
        "Teleportation",
        "Interdimensional travel"
      ]
    }
  ]
}

이 객체를 Javascript 프로그램에서 로드하고, 예를 들어 superHeroes라는 이름의 변수에 파싱하면 JavaScript object basics 문서에서 보았던 것처럼 점/브라켓 표현법을 통해 객체 내 데이터에 접근할 수 있게 됩니다. 아래와 같이요:

superHeroes.homeTown
superHeroes['active']

하위 계층의 데이터에 접근하려면, 간단하게 프로퍼티 이름과 배열 인덱스의 체인을 통해 접근하면 됩니다. 예를 들어 superHeroes의 두 번째 member의 세 번째 power에 접근하려면 아래와 같이 하면 됩니다.

superHeroes['members'][1]['powers'][2]
  1. 우선 변수 이름은 — superHeroes입니다.
  2. members 프로퍼티에 접근하려면, ["members"]를 입력합니다.
  3. members는 객체로 구성된 배열입니다. 두 번째 객체에 접근할 것이므로 [1]를 입력합니다.
  4. 이 객체에서 powers 프로퍼티에 접근하려면 ["powers"]를 입력합니다.
  5. powers 프로퍼티 안에는 위에서 선택한 hero의 superpower들이 있습니다. 세 번째 것을 선택해야 하므로 [2].

Note: We've made the JSON seen above available inside a variable in our JSONTest.html example (see the source code). Try loading this up and then accessing data inside the variable via your browser's JavaScript console.

Arrays as JSON

Above we mentioned that JSON text basically looks like a JavaScript object, and this is mostly right. The reason we said "mostly right" is that an array is also valid JSON, for example:

[
  {
    "name": "Molecule Man",
    "age": 29,
    "secretIdentity": "Dan Jukes",
    "powers": [
      "Radiation resistance",
      "Turning tiny",
      "Radiation blast"
    ]
  },
  {
    "name": "Madame Uppercut",
    "age": 39,
    "secretIdentity": "Jane Wilson",
    "powers": [
      "Million tonne punch",
      "Damage resistance",
      "Superhuman reflexes"
    ]
  }
]

The above is perfectly valid JSON. You'd just have to access array items (in its parsed version) by starting with an array index, for example [0]["powers"][0].

Other notes

  • JSON is purely a data format — it contains only properties, no methods.
  • JSON requires double quotes to be used around strings and property names. Single quotes are not valid.
  • Even a single misplaced comma or colon can cause a JSON file to go wrong, and not work. You should be careful to validate any data you are attempting to use (although computer-generated JSON is less likely to include errors, as long as the generator program is working correctly). You can validate JSON using an application like JSONLint.
  • JSON can actually take the form of any data type that is valid for inclusion inside JSON, not just arrays or objects. So for example, a single string or number would be a valid JSON object.
  • Unlike in JavaScript code in which object properties may be unquoted, in JSON, only quoted strings may be used as properties.

Active learning: Working through a JSON example

So, let's work through an example to show how we could make use of some JSON data on a website.

Getting started

To begin with, make local copies of our heroes.html and style.css files. The latter contains some simple CSS to style our page, while the former contains some very simple body HTML:

<header>
</header>

<section>
</section>

Plus a <script> element to contain the JavaScript code we will be writing in this exercise. At the moment it only contains two lines, which grab references to the <header> and <section> elements and store them in variables:

var header = document.querySelector('header');
var section = document.querySelector('section');

We have made our JSON data available on our GitHub, at https://mdn.github.io/learning-area/javascript/oojs/json/superheroes.json.

We are going to load it into our page, and use some nifty DOM manipulation to display it, like this:

Obtaining the JSON

To obtain the JSON, we are going to use an API called XMLHttpRequest (often called XHR). This is a very useful JavaScript object that allows us to make network requests to retrieve resources from a server via JavaScript (e.g. images, text, JSON, even HTML snippets), meaning that we can update small sections of content without having to reload the entire page. This has led to more responsive web pages, and sounds exciting, but it is unfortunately beyond the scope of this article to teach it in much more detail.

  1. To start with, we are going to store the URL of the JSON we want to retrieve in a variable. Add the following at the bottom of your JavaScript code:
    var requestURL = 'https://mdn.github.io/learning-area/javascript/oojs/json/superheroes.json';
  2. To create a request, we need to create a new request object instance from the XMLHttpRequest constructor, using the new keyword. Add the following below your last line:
    var request = new XMLHttpRequest();
  3. Now we need to open a new request using the open() method. Add the following line:
    request.open('GET', requestURL);

    This takes at least two parameters — there are other optional parameters available. We only need the two mandatory ones for this simple example:

    • The HTTP method to use when making the network request. In this case GET is fine, as we are just retrieving some simple data.
    • The URL to make the request to — this is the URL of the JSON file that we stored earlier.
  4. Next, add the following two lines — here we are setting the responseType to JSON, so that XHR knows that the server will be returning JSON, and that this should be converted behind the scenes into a JavaScript object. Then we send the request with the send() method:
    request.responseType = 'json';
    request.send();
  5. The last bit of this section involves waiting for the response to return from the server, then dealing with it. Add the following code below your previous code:
    request.onload = function() {
      var superHeroes = request.response;
      populateHeader(superHeroes);
      showHeroes(superHeroes);
    }

Here we are storing the response to our request (available in the response property) in a variable called superHeroes; this variable will now contain the JavaScript object based on the JSON! We are then passing that object to two function calls — the first one will fill the <header> with the correct data, while the second one will create an information card for each hero on the team, and insert it into the <section>.

We have wrapped the code in an event handler that runs when the load event fires on the request object (see onload) — this is because the load event fires when the response has successfully returned; doing it this way guarantees that request.response will definitely be available when we come to try to do something with it.

Populating the header

Now we've retrieved the JSON data and converted it into a JavaScript object, let's make use of it by writing the two functions we referenced above. First of all, add the following function definition below the previous code:

function populateHeader(jsonObj) {
  var myH1 = document.createElement('h1');
  myH1.textContent = jsonObj['squadName'];
  header.appendChild(myH1);

  var myPara = document.createElement('p');
  myPara.textContent = 'Hometown: ' + jsonObj['homeTown'] + ' // Formed: ' + jsonObj['formed'];
  header.appendChild(myPara);
}

We have called the parameter jsonObj, to remind ourselves that this JavaScript object originated from JSON. Here we first create an <h1> element with createElement(), set its textContent to equal the squadName property of the object, then append it to the header using appendChild(). We then do a very similar operation with a paragraph: create it, set its text content and append it to the header. The only difference is that its text is set to a concatenated string containing both the homeTown and formed properties of the object.

Creating the hero information cards

Next, add the following function at the bottom of the code, which creates and displays the superhero cards:

function showHeroes(jsonObj) {
  var heroes = jsonObj['members'];
      
  for (var i = 0; i < heroes.length; i++) {
    var myArticle = document.createElement('article');
    var myH2 = document.createElement('h2');
    var myPara1 = document.createElement('p');
    var myPara2 = document.createElement('p');
    var myPara3 = document.createElement('p');
    var myList = document.createElement('ul');

    myH2.textContent = heroes[i].name;
    myPara1.textContent = 'Secret identity: ' + heroes[i].secretIdentity;
    myPara2.textContent = 'Age: ' + heroes[i].age;
    myPara3.textContent = 'Superpowers:';
        
    var superPowers = heroes[i].powers;
    for (var j = 0; j < superPowers.length; j++) {
      var listItem = document.createElement('li');
      listItem.textContent = superPowers[j];
      myList.appendChild(listItem);
    }

    myArticle.appendChild(myH2);
    myArticle.appendChild(myPara1);
    myArticle.appendChild(myPara2);
    myArticle.appendChild(myPara3);
    myArticle.appendChild(myList);

    section.appendChild(myArticle);
  }
}

To start with, we store the members property of the JavaScript object in a new variable. This array contains multiple objects that contain the information for each hero.

Next, we use a for loop to loop through each object in the array. For each one, we:

  1. Create several new elements: an <article>, an <h2>, three <p>s, and a <ul>.
  2. Set the <h2> to contain the current hero's name.
  3. Fill the three paragraphs with their secretIdentity, age, and a line saying "Superpowers:" to introduce the information in the list.
  4. Store the powers property in another new variable called superPowers — this contains an array that lists the current hero's superpowers.
  5. Use another for loop to loop through the current hero's superpowers — for each one we create a <li> element, put the superpower inside it, then put the listItem inside the <ul> element (myList) using appendChild().
  6. The very last thing we do is to append the <h2>, <p>s, and <ul> inside the <article> (myArticle), then append the <article> inside the <section>. The order in which things are appended is important, as this is the order they will be displayed inside the HTML.

Note: If you are having trouble getting the example to work, try referring to our heroes-finished.html source code (see it running live also.)

Note: If you are having trouble following the dot/bracket notation we are using to access the JavaScript object, it can help to have the superheroes.json file open in another tab or your text editor, and refer to it as you look at our JavaScript. You should also refer back to our JavaScript object basics article for more information on dot and bracket notation.

Converting between objects and text

The above example was simple in terms of accessing the JavaScript object, because we set the XHR request to convert the JSON response directly into a JavaScript object using:

request.responseType = 'json';

But sometimes we aren't so lucky — sometimes we'll receive a raw JSON string, and we'll need to convert it to an object ourselves. And when we want to send a JavaScript object across the network, we'll need to convert it to JSON (a string) before sending. Luckily, these two problems are so common in web development that a built-in JSON object is available in browsers, which contains the following two methods:

  • parse(): Accepts a JSON string as a parameter, and returns the corresponding JavaScript object.
  • stringify(): Accepts an object as a parameter, and returns the equivalent JSON string form.

You can see the first one in action in our heroes-finished-json-parse.html example (see the source code) — this does exactly the same thing as the example we built up earlier, except that we set the XHR to return the raw JSON text, then used parse() to convert it to an actual JavaScript object. The key snippet of code is here:

request.open('GET', requestURL);
request.responseType = 'text'; // now we're getting a string!
request.send();

request.onload = function() {
  var superHeroesText = request.response; // get the string from the response
  var superHeroes = JSON.parse(superHeroesText); // convert it to an object
  populateHeader(superHeroes);
  showHeroes(superHeroes);
}

As you might guess, stringify() works the opposite way. Try entering the following lines into your browser's JavaScript console one by one to see it in action:

var myJSON = { "name": "Chris", "age": "38" };
myJSON
var myString = JSON.stringify(myJSON);
myString

Here we're creating a JavaScript object, then checking what it contains, then converting it to a JSON string using stringify() — saving the return value in a new variable — then checking it again.

Summary

In this article, we've given you a simple guide to using JSON in your programs, including how to create and parse JSON, and how to access data locked inside it. In the next article, we'll begin looking at object-oriented JavaScript.

See also

 

In this module

 

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