Can JSON store list?
Working with JSON
JavaScript Object Notation (JSON) is a standard text-based format for representing structured data based on JavaScript object syntax. It is commonly used for transmitting data in web applications (e.g., sending some data from the server to the client, so it can be displayed on a web page, or vice versa). You'll come across it quite often, so in this article we give you all you need to work with JSON using JavaScript, including parsing JSON so you can access data within it, and creating JSON. Show
No, really, what is JSON?JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. Even though it closely resembles JavaScript object literal syntax, it can be used independently from JavaScript, and many programming environments feature the ability to read (parse) and generate JSON. JSON exists as a string useful when you want to transmit data across a network. It needs to be converted to a native JavaScript object when you want to access the data. This is not a big issue JavaScript provides a global JSON object that has methods available for converting between the two. Note: Converting a string to a native object is called deserialization, while converting a native object to a string so it can be transmitted across the network is called serialization. A JSON string can be stored in its own file, which is basically just a text file with an extension of .json, and a MIME type of application/json. JSON structureAs described above, JSON is a string whose format very much resembles JavaScript object literal format. You can include the same basic data types inside JSON as you can in a standard JavaScript object strings, numbers, arrays, booleans, and other object literals. This allows you to construct a data hierarchy, like so: {
"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"
]
}
]
}
If we loaded this string into a JavaScript program, parsed it into a variable called superHeroes for example, we could then access the data inside it using the same dot/bracket notation we looked at in the JavaScript object basics article. For example: superHeroes.homeTown
superHeroes['active']
To access data further down the hierarchy, you have to chain the required property names and array indexes together. For example, to access the third superpower of the second hero listed in the members list, you'd do this: superHeroes['members'][1]['powers'][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 JSONAbove we mentioned that JSON text basically looks like a JavaScript object inside a string. We can also convert arrays to/from JSON. Below 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
Active learning: Working through a JSON exampleSo, let's work through an example to show how we could make use of some JSON formatted data on a website. Getting startedTo 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, plus a In our function, the first four lines use the Fetch API to fetch the JSON from the server:
Note: The fetch() API is asynchronous. We'll learn a lot about asynchronous functions in the next module, but for now we'll just say that we need to add the keyword async before the name of the function that uses the fetch API, and add the keyword await before the calls to any asynchronous functions. After all that, the superHeroes variable will contain the JavaScript object based on the JSON. We are then passing that object to two function calls the first one fills the Populating the headerNow that 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(obj) {
const header = document.querySelector('header');
const myH1 = document.createElement('h2');
myH1.textContent = obj['squadName'];
header.appendChild(myH1);
const myPara = document.createElement('p');
myPara.textContent = `Hometown: ${obj['homeTown']} // Formed: ${obj['formed']}`;
header.appendChild(myPara);
}
Here we first create an 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 template literal containing both the homeTown and formed properties of the object.Creating the hero information cardsNext, add the following function at the bottom of the code, which creates and displays the superhero cards: function populateHeroes(obj) {
const section = document.querySelector('section');
const heroes = obj['members'];
for (const hero of heroes) {
const myArticle = document.createElement('article');
const myH2 = document.createElement('h2');
const myPara1 = document.createElement('p');
const myPara2 = document.createElement('p');
const myPara3 = document.createElement('p');
const myList = document.createElement('ul');
myH2.textContent = hero.name;
myPara1.textContent = `Secret identity: ${hero.secretIdentity}`;
myPara2.textContent = `Age: ${hero.age}`;
myPara3.textContent = 'Superpowers:';
const superPowers = hero.powers;
for (const power of superPowers) {
const listItem = document.createElement('li');
listItem.textContent = power;
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...of loop to loop through each object in the array. For each one, we:
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. Calling the top-level functionFinally, we need to call our top-level populate() function: populate();
Converting between objects and textThe above example was simple in terms of accessing the JavaScript object, because we converted the network response directly into a JavaScript object using response.json(). But sometimes we aren't so lucky sometimes we receive a raw JSON string, and we need to convert it to an object ourselves. And when we want to send a JavaScript object across the network, we 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:
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:
The key snippet of code is here: async function populate() {
const requestURL = 'https://mdn.github.io/learning-area/javascript/oojs/json/superheroes.json';
const request = new Request(requestURL);
const response = await fetch(request);
const superHeroesText = await response.text();
const superHeroes = JSON.parse(superHeroesText);
populateHeader(superHeroes);
populateHeroes(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: let myObj = { name: "Chris", age: 38 };
myObj
let myString = JSON.stringify(myObj);
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. Test your skills!You've reached the end of this article, but can you remember the most important information? You can find some further tests to verify that you've retained this information before you move on see Test your skills: JSON. SummaryIn 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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