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JavaScript: undefined

It's possible to declare variable without giving it a specific value. What will be printed in this case?

let name;
console.log(name); // ?

undefined is a special value of its own type that denotes an absence of value. Undefined is used by JavaScript in many different cases, for example when calling a non-existent string character:

const name = 'Arya';

The meaning (semantics) of undefined is an absence of value. However, nothing stops you from writing the following code:

let key = undefined;

Although the interpreter allows this, it violates the semantics of undefined, because this code performs an assignment and therefore substitutes a value.

JavaScript is one of the few languages that explicitly includes undefined. Other languages use null instead of undefined, and JavaScript uses null too.

Self-check. Why can't we declare a constant without specifying a value?


Print the undefined value without explicitly specifying it (by assignment or by putting it directly into console.log()). If you can't figure out how to do it, look up the teacher's solution.

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.

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