For what purpose if(!a) statement used?

Answer 1

if (a == false)

Answer 2

If you're talking about JS, then it will check if that variable is assigned a value. Example:

var a;

if (!a) {
document.write("Variable 'a' doesn't exist.");
} else {
document.write("Variable 'a' exists.");
};

// This code would say that 'a' doesn't exist because it was never assigned a string or an integer.

Answer 3

it means your condition is not true.. '!' this symbol is use for making any boolean to false.