How do I add elements to a dynamic array and exclude exsisting elements

function addNumifnotThere(numer){
   var numCent = [];

     numCent.forEach(function(){
        if(numer in numCent)
         console.log("you logged that");
        else
         numCent.push(numer);


 });
return numCent;
}

This is my current code, what its attempting to do is read an array and if there is already an element exits the loop and says "you already logged that", obviously if it cannot find a similar element then it pushes it to the array.

I want this to work dynamically so we cannot know the size of the array beforehand, so the first element passed as an argument should be put into the array, (addNum(1) should have the array print out [1], calling addNum(1) again should print "you already logged that")

However there are two problems with this 1) Trying to push to a new array without any entries means everything is undefined and therefore trying to traverse the array just causes the program to print [].

2) Adding some random elements to the array just to make it work, in this case numCent=[1,2,3] has other issues, mainly that adding a number above 3 causes the code to print incorrect information. In this case addNum(5) should print [1,2,3,5] but instead prints [1,2,3,5,5,5]

I know this has to be a simple mistake but I've been dragging myself too long to not ask for help.

EDIT: Thanks to the many outstanding answers here I have now leanred about the indexOf method, thank you guys so much.

Solution

Do do this cleanly, I'd consider prototyping the global Array object and adding a method to push values but only if they're unique to the array. Something like this:

Array.prototype.pushUnique = function (item) {
    if (this.indexOf(item) != -1) {
        console.log("Item with value of " + item + " already exists in the array."
    }
    else {
        this.push(item);
    }
}

If you're not comfortable prototypeing global types like Array, you can build the same thing in a procedural pattern:

function arrayPushUnique (arr, item) {
    if (arr.indexOf(item) != -1) {
        console.log("Item with value of " + item + " already exists in the array."
    }
    else {
        arr.push(item);
    }
}

Then to use it, simply create a new empty array and start pushing things to it.

var numCent = [];
// The Array.prototype method
numCent.pushUnique(number);
// The procedural method
arrayPushUnique(numCent, number);