Free JavaScript course. Sign Up for tracking progress →

JavaScript: Immutability

What will the last call print?

const name = 'Tirion';
console.log(name.toUpperCase()); // => TIRION
console.log(name); // => ?

The answer to this question depends on how well you understood the lesson about the immutability of primitive data types. Calling the .toUpperCase() method returns a new value with all letters converted to uppercase but doesn't (and can't) change the original string. So the constant (or variable, it doesn't matter here) will contain the original 'Tirion' value. This logic holds true for methods of all primitive types. Moreover, attempting to change the value of a property of this data will lead to nothing:

const name = 'Tirion';
console.log(name.length); // => 6
name.length = 100;
console.log(name.length); // => 6

You can replace the value with a new one instead of changing it. This requires variables:

let name = 'Tirion';
name = name.toUpperCase();
console.log(name); // => TIRION


User input data often contains extra spaces at the end or beginning of a string. They are usually cut out with .trim(), for example, ' hello\n ' becomes 'hello'.
Update the firstName variable by assigning the same value to it, but using the .trim() method. Print the result.

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.

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