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JavaScript: Weak typing

We know two different data types: numbers and strings. For example, we can add numbers, because addition is an operation for the "numbers" type.

But what if you add a number to a string?

console.log(1 + '7'); // => '17'

Even though '7' is a string, the JavaScript interpreter returned 17 as if we were adding two strings. When JavaScript notices a type mismatch, it tries to convert the data. In our case, it converts the number 1 to a string '1' and then just concatenates '1' and '7'.

Not all languages can do this. JavaScript is a weakly typed language. It recognizes different data types (numbers, strings, etc.), but doesn't use them too strictly, trying to convert data when it seems reasonable. JavaScript sometimes even rushes into extremes. Most expressions that don't work in other languages work perfectly well in JavaScript. Try to perform any arithmetic operation (except addition) with strings or another data type (except when both operands are numbers or strings consisting only of numbers). You will see that they always work and return NaN, which makes sense.

const result = 'one' * 'two';
console.log(result); // => NaN

In strongly typed languages, adding a number to a string won't work.

JavaScript was created for the Internet, where all information is stored in strings. Even when you type a phone number or a birth year on a website, that information goes to the server as a string, not as a number. So the designers of the language decided that automatic type conversion is suitable and convenient.

This tacit automatic type conversion is indeed convenient. But in practice, this feature causes a lot of errors, which are difficult to find. The code may work or fail, depending on whether you are lucky enough for the automatic conversion to have been done correctly. A programmer may not notice this immediately.

You will encounter these cases more than once in further tasks. You'll often ask "Why doesn't my code work the way I expect?"

Weak typing runs like a thread through the whole Javascript development process.


Print the result of 7 - (-8 - -2). Convert 7 to a string. Experiment with other numbers too.

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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