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 changing the value of a variable is not a problem.
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.
