From the cases in the OP, it looks like the objective is to take a number represented by a string param, and return a string representing that number times 10.

function zero(string) {
  return `${+string*10}`
}

console.log(zero("4.0"))
console.log(zero("3"))

This happens because when you use zeroArray[1] you can get the character in that position of the string but you cannot assign a new value like that, you need an array to do this operation. Let me edit your code with some little changes:

var firstDefrost = "4.0"; 

    function zero ( zeroArray ){
        zeroArray = zeroArray.split("");
        if(zeroArray[1] == undefined){
            zeroArray[1] = "0";
            zeroArray[2] = "";
            console.log("zero array 1 called")
            }
        else if(zeroArray[1] == "."){
            zeroArray[1] = "0";
            zeroArray[2] = "";
            /*I want it to return 40 not 4.0 which is what is happening */
            console.log("zero array 2 called" + zeroArray)
            }
        
        return zeroArray.join("");
        
    }
    
firstDefrost = zero(firstDefrost);