Tutorial - Practice about symbols in JavaScript


The simplest way to create a symbol in JavaScript is to call the Symbol() function. The 2 key properties that makes symbols so special are:

  1. Symbols can be used as object keys. Only strings and symbols can be used as object keys.
  2. No two symbols are ever equal.
const symbol1 = Symbol();
const symbol2 = Symbol();

symbol1 === symbol2; // false

const obj = {};
obj[symbol1] = ‘Hello’;
obj[symbol2] = ‘World’;

obj[symbol1]; // ‘Hello’
obj[symbol2]; // ‘World’

Although the Symbol() call makes it look like symbols are objects, symbols are actually a primitive type in JavaScript. Using Symbol as a constructor with new throws an error.

const symbol1 = Symbol();

typeof symbol1; // ‘symbol’
symbol1 instanceof Object; // false

// Throws “TypeError: Symbol is not a constructor”
new Symbol();


The Symbol() function takes a single parameter, the string description. The symbol’s description is only for debugging purposes - the description shows up in the symbol’s toString(). However, two symbols with the same description are not equal.

const symbol1 = Symbol(‘my symbol’);
const symbol2 = Symbol(‘my symbol’);

symbol1 === symbol2; // false
console.log(symbol1); // ‘Symbol(my symbol)’

There is also a global symbol registry. Creating a symbol using Symbol.for() adds a symbol to a global registry, keyed by the symbol’s description. In other words, if you create two symbols with the same description using Symbol.for(), the two symbols will be equal.

const symbol1 = Symbol.for(‘test’);
const symbol2 = Symbol.for(‘test’);

symbol1 === symbol2; // true
console.log(symbol1); // ‘Symbol(test)’

Generally speaking, you shouldn’t use the global symbol registry unless you have a very good reason to, because you might run into naming collisions.

Name Collisions

The first built-in symbol in JavaScript was the Symbol.iterator symbol in ES6. An object that has a Symbol.iterator function is considered an iterable. That means you can use that object as the right hand side of a for/of loop.

const fibonacci = {
[Symbol.iterator]: function*() {
let a = 1;
let b = 1;
let temp;

yield b;

while (true) {
  temp = a;
  a = a + b;
  b = temp;
  yield b;


// Prints every Fibonacci number less than 100
for (const x of fibonacci) {
if (x >= 100) {

Why is Symbol.iterator a symbol rather than a string? Suppose instead of using Symbol.iterator, the iterable spec checked for the presence of a string property ‘iterator’. Furthermore, suppose you had the below class that was meant to be an iterable.

class MyClass {
constructor(obj) {
Object.assign(this, obj);

iterator() {
const keys = Object.keys(this);
let i = 0;
return (function*() {
if (i >= keys.length) {
yield keys[i++];

Instances of MyClass will be iterables that allow you to iterate over the object’s keys. But the above class also has a potential flaw. Suppose a malicious user were to pass an object with an iterator property to MyClass.

const obj = new MyClass({ iterator: ‘not a function’ });

If you were to use for/of with obj, JavaScript would throw TypeError: obj is not iterable. That’s because the user-specified iterator function would overwrite the class’ iterator property. This is a similar security issue to prototype pollution, where naively copying user data may cause issues with special properties like proto and constructor.

The key pattern here is that symbols enable a clear separation between user data and program data in objects. Since symbols cannot be represented in JSON, there’s no risk of data passed into an Express API having a bad Symbol.iterator property. In objects that mix user data with built-in functions and methods, like Mongoose models, you can use symbols to ensure that user data doesn’t conflict with your built-in functionality.

Private Properties

Since no two symbols are ever equal, symbols are a convenient way to simulate private properties in JavaScript. Symbols don’t show up in Object.keys(), and therefore, unless you explicitly export a symbol, no other code can access that property unless you explicitly go through the Object.getOwnPropertySymbols() function.

function getObj() {
const symbol = Symbol(‘test’);
const obj = {};
obj[symbol] = ‘test’;
return obj;

const obj = getObj();

Object.keys(obj); // []

// Unless you explicitly have a reference to the symbol, you can’t access the
// symbol property.
obj[Symbol(‘test’)]; // undefined

// You can still get a reference to the symbol using getOwnPropertySymbols()
const [symbol] = Object.getOwnPropertySymbols(obj);
obj[symbol]; // ‘test’

Symbols are also convenient for private properties because they do not show up in JSON.stringify() output. Specifically, JSON.stringify() silently ignores symbol keys and values.

const symbol = Symbol(‘test’);
const obj = { [symbol]: ‘test’, test: symbol };

JSON.stringify(obj); // “{}”

Moving On

Symbols are a great tool for representing internal state in objects while ensuring that user data stays separate from program state. With symbols, there’s no more need for conventions like prefixing program state properties with ‘$’. So next time you find yourself setting an object property to $$__internalFoo, consider using a symbol instead.

Found a typo or error? Open up a pull request! This post is available as markdown on Github

Originally published by Valeri Karpov at thecodebarbarian.com


Thanks for reading :heart: If you liked this post, share it with all of your programming buddies! Follow me on Facebook | Twitter

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Tutorial - Practice about symbols in JavaScript
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Now Learn JavaScript Programming Language With Microsoft

icrosoft has released a new series of video tutorials on YouTube for novice programmers to get a hands-on renowned programming language — JavaScript.

This isn’t the first attempt by Microsoft to come up with video tutorials by beginner programmers. The company also has a series of YouTube tutorials on Python for beginners.

For JavaScript, Microsoft has launched a series of 51 videos as ‘Beginner’s Series to JavaScript,’ for young programmers, developers and coders who are interested in building browser applications using JavaScript. These video tutorials will also help programmers and coders to use relevant software development kits (SDKs) and JavaScript frameworks, such as Google’s Angular.

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JavaScript Snake Game Tutorial - Develop a Simple Snake Game

Work on real-time JavaScript Snake game project and become a pro

Snake game is an interesting JavaScript project for beginners. Snake game is a single-player game, which we’ve been playing for a very long time. The game mainly consists of two components – snake and fruit. And we just need to take our snake to the food so that it can eat and grow faster and as the number of fruits eaten increases, the length of snake increases which makes the game more interesting. While moving if the snake eats its own body, then the snake dies and the game ends. Now let’s see how we can create this.

JavaScript Project Prerequisites

To implement the snake game in JavaScript you should have basic knowledge of:

1. Basic concepts of JavaScript


3. CSS

Download Project Code

Before proceeding ahead please download source code of Snake Game: Snake Game in JavaScript

Steps to Build the Project – JavaScript Snake game

1. Create Html file

HTML builds the basic structure. This file contains some basic HTML tags like div, h1, title, etc. also we’ve used bootstrap (CDN is already included).



<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>DataFlair Snake game</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css" integrity="sha384-9aIt2nRpC12Uk9gS9baDl411NQApFmC26EwAOH8WgZl5MYYxFfc+NcPb1dKGj7Sk" crossorigin="anonymous">
    <link rel="stylesheet" href="static/style.css">
    <div class="container">
        <div class ="Jumbotron">
        <h1>DataFlair Snake game using vanilla JavaScript</h1>

            <h2 class="btn btn-info"> 
                Score:   0 

            <div class="containerCanvas">
                <canvas id="canvas" width="500" height="500" class="canvasmain"> </canvas>
    <script src="static/fruit.js"></script>
    <script src="static/snake.js"></script>
    <script src="static/draw.js"></script>

We have used simple HTML tags except tag. This new tag was introduced in HTML5, which is used to draw graphics, via JavaScript. It is nothing without JavaScript so we will use it in our JavaScript file to actually draw the board and the objects required. We’ll see its implementation in JS files.

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