# JavaScript: Combination of logical expressions

Logical operations are expressions. So, you can combine them with other expressions.

Let's say, for example, we want to check if a number is even, i.e., a multiple of two. In programming we do this:

• check for a remainder when dividing by 2:
• if the remainder is 0, then the number is even
• if the remainder is not 0, then the number is odd

The remainder of a division is an elementary but crucial concept of arithmetic, algebra, and even number theory and cryptography. The idea is simple: divide a number into several equal groups, and if anything remains, it's the remainder of the division.

Split some candies equally among individuals:

• 7 candy, 2 persons: 2 x 3 + remainder 1. So 7 is not a multiple of 2.
• 21 candies, 3 persons: 3 x 7 + remainder 0. So 21 is a multiple of 3.
• 19 candies, 5 persons: 5 x 3 + remainder 4. So 19 is not a multiple of 5.

The `%` operator calculates the remainder of a division (not to be confused with division itself):

``````7 % 2;  // 1
21 % 3; // 0
19 % 5; // 4

// Checking if even

10 % 2 // 10 is even, i.e. remainder is 0
9 % 2  // 9 odd, i.e. remainder is 1
``````

Write a function to check whether a number is even:

``````const isEven = (number) => number % 2 === 0;

isEven(10); // true
isEven(3);  // false
``````

In one expression, we've combined the logical operator `===` (equality check) and the arithmetic operator `%`.

Arithmetic operators have higher priority than logical ones. So, we first calculate the arithmetic expression `number % 2`, and then its output is used in the logical comparison.

Literally, it means the following: "calculate the remainder of dividing `number` by 2 and check if the remainder equals zero; then return the result of the equality check ".

Another example: write a function to check the first letter of a string is uppercase.

Algorithm:

1. Get the initial character of the string and assign it to the variable
2. Compare whether the character is equal to its uppercase version
3. Return the result
``````const isFirstLetterInUpperCase = (string) => {
const firstLetter = string;
return firstLetter.toUpperCase() === firstLetter;
};

isFirstLetterInUpperCase('marmont'); // false
isFirstLetterInUpperCase('Robb');    // true
``````

https://replit.com/@hexlet/js-basics-logica-operations

## Instructions

Write a function to check the format of a given phone number. If the number starts with +, then it's in the international format.

``````isInternationalPhone('89602223423'); // false
isInternationalPhone('+79602223423'); // true
``````
The exercise doesn't pass checking. What to do? 😶

• 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 🙄

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