Curriculum

JavaScript Basic

JSHome

0/216

JS Versions

0/36

JS Objects

0/15

JS Funtions

0/22

JS Classes

0/3

JS Async

0/10

JS HTML DOM

0/25

JS Browser BOM

0/17

JS Web APIs

0/7

JS AJAX

0/11

JS JSON

0/16

JS vs jQuery

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JS Graphics

0/7

Text lesson

The Symbol Type

A JavaScript Symbol is a primitive data type, similar to Number, String, or Boolean.

It represents a unique, “hidden” identifier that cannot be accidentally accessed by other code.

For example, if multiple developers want to add a person.id property to a person object in third-party code, their values might conflict. Using Symbol() to create unique identifiers resolves this issue.

Example

const person = {
firstName: “John”,
lastName: “Doe”,
age: 50,
eyeColor: “blue”
};

let id = Symbol(‘id’);
person[id] = 140353;
// Now person[id] = 140353
// but person.id is still undefined

Note:
Symbols are always unique.

Even if you create two symbols with the same description, they will have distinct values.

Symbol(“id”) == Symbol(“id”); // false

Default Parameter Values

ES6 allows you to assign default values to function parameters.

Example

function myFunction(x, y = 10) {
// y is 10 if not passed or undefined
return x + y;
}
myFunction(5); // will return 15

Function Rest Parameter

The rest parameter (…) allows a function to collect an indefinite number of arguments into an array.

Example

function sum(...args) {
  let sum = 0;
  for (let arg of args) sum += arg;
  return sum;
}

let x = sum(4, 9, 16, 25, 29, 100, 66, 77);

String.includes()

The includes() method returns true if a string contains a specified value, and false otherwise.

Example

let text = “Hello world, welcome to the universe.”;
text.includes(“world”) // Returns true

String.startsWith()

The startsWith() method returns true if a string starts with a specified value, and false otherwise.

Example

let text = “Hello world, welcome to the universe.”;

text.startsWith(“Hello”) // Returns true

String.endsWith()

The endsWith() method returns true if a string ends with a specified value, and false otherwise.

Example

var text = “John Doe”;
text.endsWith(“Doe”) // Returns true

Array entries()

Example

Create an Array Iterator and iterate over the key/value pairs using it.

const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
const f = fruits.entries();

for (let x of f) {
document.getElementById(“demo”).innerHTML += x;
}

The entries() method returns an Array Iterator object containing key/value pairs:

[0, “Banana”]
[1, “Orange”]
[2, “Apple”]
[3, “Mango”]

The entries() method does not modify the original array.

Array.from()

The Array.from() method creates an Array object from any object that has a length property or from any iterable object.

Example

Convert a string into an array:

Array.from(“ABCDEFG”) // Returns [A,B,C,D,E,F,G]

Array keys()

The keys() method returns an Array Iterator object containing the keys (indexes) of an array.

Example

Create an Array Iterator object that contains the keys (indexes) of the array.

const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
const keys = fruits.keys();

let text = “”;
for (let x of keys) {
text += x + “<br>”;
}

Array find()

The find() method returns the value of the first array element that satisfies a given test function.

In this example, it returns the first element that is greater than 18.

Example

const numbers = [4, 9, 16, 25, 29];
let first = numbers.find(myFunction);

function myFunction(value, index, array) {
return value > 18;
}

Note that the function accepts three arguments:

The item value
The item index
The array itself