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JavaScript: Data types

What happens if we try to multiply a number by a string? JavaScript will return the NaN (not a number) value we've seen before. This happens whenever incompatible values are used together. In this case, a number and a string:

3 * 'Dracarys'; // NaN

In high-level programming languages, data are categorized by type. A string refers to the String type, while numbers refer to Number and BigInt (very large numbers). What are these data types for? They help to protect your program from hard-to-find errors. Types determine two things:

  • Possible values. For example, numbers in JavaScript are divided into two types: Number and BigInt. All numbers below a certain threshold (you can check it) belong to the Numbers data type, and all numbers above it belong to the Biglnt type. They're divided this way due to the hardware's technical features.
  • A set of operations applied to this data type. For example, you can multiply integers, but not strings. Multiplying the word "mother" by the word "notepad" makes no sense.

Javascript may act in one of two possible ways when it sees a violation. In some situations, it'll terminate the program with an error. In others, the program will continue to work, though an invalid operation will return something like NaN as in the example above.

How does JavaScript detect the data type? It's quite simple. Any value is initialized somewhere and, based on an initialization method, it understands what type of data it is. Numbers, for instance, are just numbers without any extra characters, apart from the point (.) for rational numbers. Strings, on the other hand, always require enclosing with special characters (there are three ways to write strings in JavaScript). For example, '234' is a string, even though there are numbers in quotes.

You can find out a data type using typeof operator:

typeof 3; // 'number'
typeof 'Game'; // 'string'

The Number, BigInt, and String data types are primitive types. But there is more. JavaScript has a built-in composite type Object (as well as arrays, dates and others based on it). It serves to combine data of different types into a single value, e.g. we can create a user combining his name and age.

// You can learn this notation on Hexlet
const user = { name: 'Toto', age: 33 };


Print the number -0.304.

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.



  • Data type - is a set of data in code. For example, integers, rational numbers, and strings are different data types. The data type determines what can be done with the elements of a particular data.

  • Primitive data types - are simple types built into the programming language.

  • A string - is a data type defining a set of characters (text). For example, 'text' or "text".

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