How to check if a variable has increased (JavaScript)

I want my code to check if a variable has increased. For example, I have a function that looks for the letter A in a text box, which runs every time a key is pressed. Then it counts the number of A's it has found. Then I want to alert me everytime a new A is written. I thought I could do this easily by just checking if the var numberOfA has increased in an if statement, and if it has to alert me, but I can't figure it out. I have tried using ++ and =/== in my if statement, but it alerts me every single time a letter is typed, instead of only everytime an A has been typed. All help is appreciated, I'm pretty new at this.

function checkForA(){
var sentence = document.getElementById("userText").value.match(/a/g);
var numberOfA = sentence.length;

if (numberOfA = numberOfA +1) {
 alert(" The letter A has been typed ");
 } 
}