Let's continue our exploration of data sets in JavaScript by having a look at the objects!
An object is also a collection of data but in a structured way as it's stored by pairs of property (or key) / value. For instance:
const BOOK = {
title: "The Lord of the Rings",
"volumes": 3,
"main characters": ["Frodo", "Aragorn"],
9: ["Fellows", "Nazgul"]
};
Notes:
To access the propertie's value, you can either use the dot or the bracket notation:
const TITLE = BOOK.title;
const VOLUMES = BOOK["volumes"];
const MAIN_CHARACTERS = BOOK["main characters"];
const NINE = BOOK[9]
You can use the dot notation if your property in a string of a single word. Otherwise, user the bracket notation with quotes for strings and without for numbers.
Tip: not sure a property exists? Use optionnal chaining (?.)! It helps prevent errors when accessing nested properties that might be null or undefined.
console.log(BOOK?.isbn);
Properties stored as value of a variable can also be access with the bracket notation. For instance:
const MAIN_CHARACTERS = "main characters";
let charactersList = BOOK[MAIN_CHARACTERS];
console.log(charactersList);
will print
[ 'Frodo', 'Aragorn' ]
on the console.
This can be very useful for iterating through an object's properties.
The same principle applies to access to sub propertie's values, just continue to filter via bracket or dot notation until you reach your destination. For instance (from FreeCodeCamp):
const ourStorage = {
"desk": {
"drawer": "stapler"
},
"cabinet": {
"top drawer": {
"folder1": "a file",
"folder2": "secrets"
},
"bottom drawer": "soda"
}
};
ourStorage.cabinet["top drawer"].folder2; // returns string "secrets"
ourStorage.desk.drawer; // returns string "stapler"
To check if the property of a given object exists or not, use the .hasOwnProperty(propname) method. It returns true or false if the property is found or not. For instance:
BOOK.hasOwnProperty("title");
BOOK.hasOwnProperty("author");
The first hasOwnProperty returns true, while the second returns false.
Note: always use quotes for string properties, otherwise an error is thrown.
Use Object.keys() method. It takes an object as the argument and returns an array of strings representing each property in the object. The order is first all non-negative integer keys in ascending order by value, then other string keys in ascending chronological order of property creation. For instance:
Object.keys(BOOK) // returns [ '9', 'title', 'volumes', 'main characters' ]
Same principle. Use Object.values() method.
Object.values(BOOK) // returns [ [ 'Fellows', 'Nazgul' ], 'The Lord of the Rings', 3, [ 'Frodo', 'Aragorn' ] ]
Same principle. Use Object.entries() method.
Object.entries(BOOK) //returns [ [ '9', [ 'Fellows', 'Nazgul' ] ], [ 'title', 'The Lord of the Rings' ], [ 'volumes', 3 ], [ 'main characters', [ 'Frodo', 'Aragorn' ] ] ]
Use a for...in statement. It looks like this:
for (let item in BOOK) {
console.log(item);
} // logs 9, title, volumes, main characters, each value in its own line
Here, "item" is a variable we define - it could be any name. Its value will change at each iteration.
The example comes from FreeCodeCamp.
Consider this object:
const user = { name: 'John Doe', age: 34 };
To extract both values and assign them to variables you could use:
const { name, age } = user;
Read it as: "create constants named "name" and "age" containing values from "name" and "age" properties in "user" object". This is a shorthand equivalent of
const name = user.name;
const age = user.age;
A console.log on name and age would return John Doe and 34, each on its own line
const declaration can by bypassed. To protect data from mutation, use Object.freeze() function. The argument would be the object you'd like to freeze. For instance (from FreeCodeCamp):
let obj = {
name:"FreeCodeCamp",
review:"Awesome"
};
Object.freeze(obj);
obj.review = "bad";
obj.newProp = "Test";
obj.review and obj.newProp assignments will result in errors and the object wouldn't change.
Proceed like any other variable; using dot or bracket notation. For instance:
BOOK["main characters"][1] = "Sam";
BOOK["main characters"][2] = "Bill";
console.log(BOOK["main characters"]);
will print
[ 'Frodo', 'Sam', 'Bill' ]
on the console.
Proceed just like you would for updating. For instance:
BOOK.format = "";
BOOK["number of copies"] = 100;
BOOK[5] = "Hello";
BOOK object will now look like this:
{
'5': 'Hello',
'9': [ 'Fellows', 'Nazgul' ],
title: 'The Lord of the Rings',
volumes: 3,
'main characters': [ 'Frodo', 'Aragorn' ],
format: '',
'number of copies': 100
}
Order has not much importance from an object perspective because you're not using indexes to access data. But if this is important, consider using the spread operator (I'll probably write a blog entry later on this subject). Note that number properties order themselves at the beginning of the object.
User the delete keyword and then target your property according to it's format. For instance:
delete BOOK.format;
delete BOOK["number of copies"];
delete BOOK[5];
I learned a few things about JavaScript on OpenClassrooms (taking Apprenez à programmer avec JavaScript & Écrivez du JavaScript pour le web courses). I didn't know it then but I was actually looking for DOM manipulation for my Qwixx project. Anyway, I'm refreshing my newbie knowledge by taking FreeCodeCamp JavaScript Algorithms and Data Structures course and here is a recap about arrays.
An array can store multiple pieces of data in a single variable. Each entry is separated by a comma and nested into square brackets. Just like this:
const SANDWICH = ["peanut butter", 1, true, ["bakery", "home-made"]];
An array containing another array is also called a multi-dimensional array. An array can also contain objects.
| "peanut butter" | 1 | true | ["bakery", "home-made"] |
| 0 | 1 | 2 | 3 |
Arrays use zero-based indexing, so the first element in an array has an index of 0. Use square bracket notation to get the data associated to the index. For instance:
console.log(SANDWICH[0]);
const EATEN = SANDWICH[1];
console.log(EATEN);
will print "peanut butter" string and 1 number.
Additional pair of brackets refers to the next level of entries inside.
const BOUGHT_IN = SANDWICH[3][0];
console.log(BOUGHT_IN);
will print "bakery".
JavaScript handles associative arrays
let currencies = [];
currencies["EUR"] = "Euro";
currencies["USD"] = "American dollar";
console.log(currencies); // [EUR: 'Euro', USD: 'American dollar']Use indexOf() method. It returns -1 if the item is not found. For instance:
SANDWICH.indexOf('peanut butter'); // returns 0
SANDWICH.indexOf('butter'); // returns -1
Use spread operator (...). For instance (from FreeCodeCamp):
const arr = [6, 89, 3, 45];
const maximus = Math.max(...arr); // maximus has the value of 89
Use a for loop. It looks like this:
let array = [1, 2, 3]
for (let i = 0; i < array.length; i++) {
console.log(array[i])
} // logs 1, 2, 3, each on its own line
Even if an array is defined as a const its entries are mutable. From our example,
SANDWICH[0] = "houmus";
SANDWICH now has the value
["houmus", 1, true, ["bakery", "home-made"]];
Use push() method. It takes one or more parameters and "pushes" them onto the end of an array. For instance:
const arr = [1, 2, 3];
arr.push(4);
arr now has the value [1, 2, 3, 4]
Use unshift() method. For instance:
const arr = [1, 2, 3];
arr.unshift(0);
arr now has the value [0, 1, 2, 3]
Use splice() method.
First argument: the index at which to begin deleting elements
Second argument (optional): the number of elements to be deleted
Third argument (optional): items to insert starting at that same index, searated by a comma
For instance (example from FreeCodeCamp):
const numbers = [10, 11, 12, 12, 15];
const startIndex = 3;
const amountToDelete = 1;
numbers.splice(startIndex, amountToDelete, 13, 14);
console.log(numbers); // returns [ 10, 11, 12, 13, 14, 15 ]
User spread operator (...). For instance (example from FreeCodeCamp):
let arrayToAppend = ['sage', 'rosemary', 'parsley', 'thyme'];
let mainArray = ['basil', 'cilantro', ...arrayToAppend, 'coriander'];
mainArray now has the value ['basil', 'cilantro', 'sage', 'rosemary', 'parsley', 'thyme', 'coriander']
Use pop() method. For instance:
const arr = [1, 2, 3];
arr.pop();
arr now has the value [1, 2]
Use shift() method. For instance:
const arr = [1, 2, 3];
arr.shift();
arr now has the value [2, 3]
Note: removed data can be returned if assigned to a variable. For instance:
const arr = [1, 2, 3];
let removedNumber = arr.pop();
let one = arr.shift();
removedNumber has the number 3 as a value and one, 1
You can also use splice() method!
First argument: the index at which to begin deleting elements
Second argument (optional): the number of elements to be deleted
For instance (from FreeCodeCamp):
let array = ['today', 'was', 'not', 'so', 'great'];
array.splice(2, 2); // array now has the value ['today', 'was', 'great']
Here we remove 2 elements, beginning with the third element (at index 2). array would have the value
Note: removed data can be returned if assigned to a variable.
I realized it was not working like classic variables like these:
let a = "first variable"; // a contains "first variable" string
let b = a; // a and b contain "first variable" string
b = "second variable"; // a contains "first variable" and b "second variable" string
let array1 = [1, 2, 3]; // array1 contains [1, 2, 3] array
let array2 = array1; // array1 and array2 contain [1, 2, 3] array
array2.unshift(0); // array1 and array2 contain [0, 1, 2, 3] array
Here, array2 behaves just like another pointer to the same object. Modifying array2 will modify array1 and inversely. So how can we copy an array? Here are several methods:
For instance:
let originalArray = ['great', 'awesome', 'super', 'wonderful'];
let copiedArray = [...originalArray];
originalArray remains unchanged and copiedArray has the same elements as originalArray.
With slice() method, we can copy or extract a given number of elements to a new array, leaving the original array unchanged. It takes 2 parameters:
First argument: the index at which to begin extraction
Second argument (optional): the index at which to stop extraction (not included)
For instance (example from FreeCodeCamp):
let weatherConditions = ['rain', 'snow', 'sleet', 'hail', 'clear'];
let todaysWeather = weatherConditions.slice(1, 3); // todaysWeather is ['snow', 'sleet']
some() methods accepts a callback function which should take an element of the array as the argument. It will will return true if the callback function returns true for at least one element in the array.