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JavaScript: Some errors regarding variables

The order of statements in code with variables is tremendously important. You must declare a variable before using it. Below is an example of a common beginner's mistake:

// Uncaught ReferenceError: greeting is not defined
let greeting = 'Father!';

Running the above program terminates with the error ReferenceError: greeting is not defined. ReferenceError means that the code uses a name (called an identifier) that is not defined. And the error itself explicitly tells you what it is: greeting is not defined. Besides incorrect definition order, there are more common typos in JavaScript, both when using a variable and when declaring it.

Using a properly configured editor can reduce the number of such errors. These kinds of editor highlight names used without being declared and warns about possible problems.

Another common mistake is trying to declare a variable that is already declared.

let greeting = 'Father!';
let greeting = 'Father!';

You can't do that. You have to create a new variable.


Find an undeclared variable in the code and declare it by assigning the value 'Dragon';

The result should look like this:

The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

By the way, you can participate in courses improvement. There is a link below to the lessons course code which you can edit right in your browser.


  • Variable is a way to save data under a name for later use in code.

If you got stuck and don't know what to do, you can ask a question in our community