Negation

In JavaScript, the ! operator is used to negate a value. We've already seen an example of negation at work:

• !== is the not equal operator

We can also negate a Boolean value using the ! operator. Negating a Boolean just means taking its opposite:

• !true (NOT true) is false
• !false (NOT false) is true

Run the starter code to see Boolean negation in practice.

Does NOT include

A few times now we've used the include method in a conditional. For example, when testing if the first letter of a word is a vowel, we used this conditional:

if ("aeiouAEIOU".includes(firstLetter)) { ... }

When we did, we were asking if firstLetter -- whatever its value -- could be found in the string containing all the vowels.

What if instead we wanted to know when a character could NOT be found in a string? We could use the ! operator to negate the result of the includes, like so:

if (!"aeiouAEIOU".includes(firstLetter)) { ... }

Tests

Let's write a different sort of validation function, a variable name validator. We know that JavaScript variables cannot start with a number. To pass the tests, write a function called isValidVariableName that takes a string (the variable name) and returns true if the variable name does NOT start with a number and returns false otherwise.

HINT:

• Try to use the ! operator to negate the result of the includes method in your solution.