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You will definitely, like the rest of us, make mistakes in the code. Software bugs are called bugs. Bug - means an error in the program💾 or in the system, due to which the program produces unexpected behavior and, as a result, the result. Most software errors arise from errors made by program developers in its source code or in its design.


In the meaning of "elusive technical error", the word "bug" was used long before the advent of computers by the staff of telegraph and telephone companies in relation to problems with electrical equipment and radio equipment. In 1878, Thomas Edison wrote:

“This was the case with all my inventions. 1️⃣ The first step is intuition, which comes like a flash, then difficulties arise - the device refuses to work, and that's when the bugs appear - as these small mistakes and difficulties are called - and it takes months of close observation, research and effort before it comes to commercial success or failure. "

Debugging is the process of finding and fixing errors in a script.


The most common mistakes


I hope you have already encountered your first mistakes in the process of writing code. Why hope? Because mistakes are our teachers who show us what we do wrong in our code and a computer ️, or rather a code interpreter, simply cannot understand us. Mistakes when writing code happen almost every day. The trick is to be able to read the error message that the machine will give you in order to quickly find and fix a defect in the written code. The more you learn JavaScript, the more you appreciate the error messages - they often show very accurately where you went wrong.

A couple of the most common error types in code код:



Syntax Error - violation of language rules правил. For example, enter nine plus a semicolon 9 +; :

Live Editor

Reply: SyntaxError: Unexpected token;

A syntax error simply means that there is an error in the body of your sentence. In other words, what you wrote is not correct in terms of JavaScript. The interpreter cannot read your sentence and does not know what to do with it. Unexpected token; means that the interpreter has read something that it did not expect to read at all: in our case, a semicolon ;.

Let's make a mistake again! Let's enter five plus three in the console and a parenthesis at the end of 5 + 3).

Live Editor

You have a closing parenthesis ), but there is no opening parenthesis (! But the parentheses always go in pairs - it cannot be that there is a closing, but there is no opening parenthesis, and vice versa.



ReferenceError is an invalid name! The ReferenceError object represents an error that occurs when accessing a variable that does not exist. For example, enter five plus alternating 5 + alternating:

Live Editor

Now we have before us an error of the reference ReferenceError. Perhaps you have already noticed what is the matter here? Let's read the error message carefully (after all, that's why it is displayed!). It says: variable is not defined - the variable is not set, this is where our problem lies! First, we have to declare the variable-in some way, say this way:

Live Editor


A TypeError object represents an error that is thrown when a value is not of the expected type. We apply the toUpperCase method, which we will learn more about later, to the undefined type, and this is not allowed since this method converts the string to uppercase. Check this error in the browser console, it doesn't work in LIVE EDITOR.

var foo = undefined


Open source

We can also make mistakes, so if you find an error on the site or incorrect translation, you can easily help fix the error on the site. To do this, click on the Edit this page button at the very bottom of each page.




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What does Syntax Error mean?

  1. Wrong order of signs
  2. Violation of the rules of the language
  3. Inappropriate parentheses

What does Reference Error mean?

  1. Undeclared variable
  2. Syntax error
  3. Wrong name

In order to understand how much you learned this lesson, take the test in the mobile application of our school on this topic or in our telegram bot.


  1. MDN web docs
  2. Code for Teens: The Perfect Beginner's Guide to Programming, Volume 1: Javascript - Jeremy Moritz

Contributors ✨

Thanks goes to these wonderful people (emoji key):

Dmitriy Vasilev


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