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JavaScript: Primitive data types immutability

What happens if you change a character in a string?

let firstName = 'Alexander';
// the code will run correctly
firstName[0] = 'B';
console.log(firstName); // => 'Alexander'

Oddly, the value of the firstName variable remains the same, and the code is executed without errors. This is possible due to the immutability of primitive types in JavaScript. There are no ways to change a string itself. The immutability of primitives is important for many reasons, and performance is a key one. But how do we edit a string? We have variables for this purpose:

let firstName = 'Alexander';
// the code will run correctly
firstName = 'Blexander'
console.log(firstName); // => 'Blexander'

There is a big difference between changing the value of a variable and the value itself. You can't change primitive types in JavaScript (but you can change composite types, learn more about this on Hexlet), but changing the value of a variable is not a problem.


Here are three constants with different people's names. Make up and print a word out of the characters in this order:

  1. The third character from the first line.
  2. The second character from the second line.
  3. The fourth character from the third line.
  4. The fifth character from the second line.
  5. The third character from the second line.

Try using interpolation: you can put individual characters (called via square brackets) inside curly brackets, as well as variables.

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 🙄

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