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# JavaScript: Logical operators

You can combine logical expressions to create increasingly cleverer and more useful checks. One good example is password verification. As you know, some websites want a password of 8 to 20 characters on signup. Frankly, it's a weird restriction, but whatever, it is what it is. In math, we would write `8 < x < 20` (where `x` is the length of a particular password), but that trick won't work in JavaScript. We would have to make two separate logical expressions and connect them with the special "AND" operator:

``````A password longer than 8 characters **AND** a password shorter than 20 characters.
``````

Here's a function that takes the password and says whether it meets the conditions or not:

``````const isStrongPassword = (password) => {
const length = password.length;
return length > 8 && length < 20;
};

``````

`&&` means "AND" (called a conjunction in mathematical logic). The whole expression is true only when every operand, which are all part of the compound expressions, is true. In other words, `&&` means "both".

This operator's priority is lower than that of comparison operators, so the expression works correctly without parentheses.

Another widespread operator along with `&&` is `||` — "OR" (disjunction). It means "one or the other, or both". Operators can be combined in any number and any sequence, but when `&&` and `||` appear together, you should label priority with parentheses. Below is an example of an advanced function validating a password:

``````const hasSpecialChars = (str) => /* checks for special characters in the string */;

const length = password.length;
// The parentheses set the priority, making it clear how each part is related
return (length > 8 && length < 20) || hasSpecialChars(password);
};
``````

Another example. We want to buy an apartment that meets these conditions: an area of 100 square meters or more on any street OR an area of 80 square meters or more, but on `Main Street`.

We'll write a function checking the apartment. It takes two arguments: the area (a number) and the street name (a string):

``````const isGoodApartment = (area, street) => {
// Via a variable to make sure the function is not too long
const result = area >= 100 || (area >= 80 && street === 'Main Street');
return result;
};

isGoodApartment(91, 'Queens Street'); // false
isGoodApartment(78, 'Queens Street'); // false
isGoodApartment(70, 'Main Street');   // false

isGoodApartment(120, 'Queens Street'); // true
isGoodApartment(120, 'Main Street');   // true
isGoodApartment(80, 'Main Street');    // true
``````

The area of mathematics dealing with logical operators is called Boolean algebra. The "truth tables" are shown below and can be used to figure out the result of an operator:

## AND `&&`

A B A && B
TRUE TRUE TRUE
TRUE FALSE FALSE
FALSE TRUE FALSE
FALSE FALSE FALSE

Few examples:

``````// true && true;
3 > 2 && 'wow'.startsWith('w'); // true

// true && false;
'start' === 'start' && 8 < 3; // false
``````

## OR `||`

A B A || B
TRUE TRUE TRUE
TRUE FALSE TRUE
FALSE TRUE TRUE
FALSE FALSE FALSE

Few examples:

``````// true || true;
3 > 2 || 'wow'.startsWith('w'); // true

// false || false;
'start' === 'Start' || 3 < 3; // false
``````

## Instructions

Write a function, `isLeapYear()`, to determine whether a year is a leap year or not. A leap year is a multiple of 400 (i.e. divisible without a remainder), or it is both a multiple of 4 and not a multiple of 100. As you can see, the definition already contains all the required logic, all we need to do is to put it into code:

``````isLeapYear(2018); // false
isLeapYear(2017); // false
isLeapYear(2016); // true
``````

You can test multiplicity as follows:

``````// % - returns the remainder after dividing the left operand by the right one
// Check if number is a multiple of 10
number % 10 === 0

// Check if number is not a multiple of 10
number % 10 !== 0
``````
The exercise doesn't pass checking. What to do? 😶

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In my environment the code works, but not here 🤨

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My code is different from the teacher's one 🤔

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## Definitions

• Logical operators are the AND (&&) and OR (||) operators, which allow you to create compound logical conditions.

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