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JavaScript: Determinacy

Regardless of the programming language, functions possess certain fundamental properties. Knowing these properties makes it easier to predict the behavior of functions, as well as their testing and their usage. These properties include determinacy. A function is deterministic when it returns the same result for the same input parameters. For example, a function counting the number of characters is deterministic:

import { length } from 'hexlet-basics/string';

length('hexlet'); // 6
length('hexlet'); // 6

length('wow'); // 3
length('wow'); // 3

No matter how many times we call this function and pass 'hexlet', it will always return 6. In turn, a function that returns a random number is not deterministic, as the same input (even if it is empty, i.e. without parameters) will always output a different result. How it differs doesn't matter, even if at least one of a million calls returns something different, this function is deemed non-deterministic.

// A function that returns a random number
Math.random(); // 0.09856613113197676
Math.random(); // 0.8839904367241888

So what use is knowing that to us? Determinacy seriously affects many different aspects. Deterministic functions are easy to work with, easy to optimize, and easy to test. If you can make a function deterministic, it's best to make it one.


The function Math.random() returns a random number from 0 to 1 with many decimal places. But in real tasks, you sometimes want to get random integers, for example, a random number 0 to 10. Write a program printing specifically those numbers. To solve it you'll need Math.random() and Math.round().

Try to solve this task using only 1 line.


Since Math.random() only returns numbers between 0 and 1, we need to multiply it by 10 to get numbers between 0 and 10. Then we round the result to get what we need.

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.

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