Results of logical operations — JavaScript fundamentals

Look at the code below and try to guess what it will print.

console.log(0 || 1);

Correct answer: 1.

The OR operator works in such a way that its execution (left to right) interrupts the first argument and returns the result, converting it into a value of true.

Example:

console.log(0 && 1); => // 0

The AND operator works in such a way that its execution (left to right) interrupts the first argument and returns the result, converting it into a value of false.

There are two basic rules in JavaScript by which conversions take place:

• 0, '', undefined, NaN, and null are converted into false. Those values are considered falsy
• Everything else is converted into true

Developers use this extensively, for example, to define a default value:

const value = name || '';
// Examples
234 || ''; // 234
'hexlet' || ''; // 'hexlet'
undefined || ''; // ''

If name is given one of the falsy values, an empty string will be assigned to the value constant. In that case, in subsequent code, we will be able to treat value as a string.

But there is a potential bug. If name contains a falsy value and it's okay to assign values like 0, undefined, NaN or null to the value constant, then the code above will get it wrong:

// Oops
false || ''; // ''
0 || ''; // ''
undefined || ''; // ''

We covered the comparison operators === and !== in one of our lessons and we mentioned that JavaScript also has the operators == and !=, though you shouldn't use them. The very difference lies in the type conversion:

console.log('' === false); // => false
console.log('' == false);  // => true

An empty string and false are different values, so the === operator says "False! They are not equal!".

But the == operator converts types, making an empty string and false equivalent from its point of view.

This conversion is implicit, so avoid the == and != operators whenever possible.

Remember the negation operator:

const answer = true;
console.log(!answer); // => false

When you use double negation !! the result value is equal to the initial value:

const answer = true;
console.log(!!answer); // => true

However, this is where type conversions also take place. So the result of the double negation will always be a boolean value. This trick sometimes helps change the data type.

Selection error

Imagine a task to check if a value is equal to either one thing or another. For instance, the variable value must contain one of two values: first or second. Novice developers sometimes write this expression this way:

value === ('first' || 'second')

That's how we picture it in our minds, yet languages work differently, so this kind of code will lead to the wrong result. How do you read it correctly? We have to remember operator priority. The first thing to evaluate is wrapped in parentheses, i.e. 'first' || 'second'. If we execute this code, the output will be:

node
'Welcome to Node.js v17.4.0.
> 'first' || 'second'
'first'
>

Now we can substitute the original expression with the partly evaluated one:

value === 'first'

Not what we expected at all. Now, back to the start. Let's write the test correctly:

// Parentheses are not necessary,
// because === precedence is higher than that of ||
value === 'first' || value === 'second'