Reset the amended values in the Else part of an if/else statement by removing the function

Is there an equivalent in JS of the initial value in CSS. When you have a function that you want to behave in a certain way with an if statement but, then in the else part you want the values to just be what they they were before the if piece of code returned true. I've always re-written the original values as part of the else, but this seems to be a galatically inefficient way of doing things, for example:

var a = something, b = something_else;

if (a) {
run a function which changes lots of values;
} else {
    re-type the values to what they were before the function ran;
}

A more concrete version of what I'm trying to do is below. I have a forEach method that changes some string values. If I want to set it so that on the else all the code in the initial if is ignored I know I can do this by copy and pasting the code under a different function name and setting the second slice value to 300, which is the length of the original strings, but this seems a very verbose way of doing things?

There must be a way of setting the else code so it removes / kills off the original myresize() function so all the original values hold true?

var content = document.querySelectorAll(".generic-content p");

function textSlice() {
    if (window.innerWidth < 500) {
        function myresize() {

        content.forEach(function(index) {
            var x2, x3, x4;
            x2 = index.textContent;
            x3 = x2.slice(0, 100) + "[...]";
            index.textContent = x3;

        });
            myresize();
    }


    } else {
        // remove the myresize(); function or somehow kill it
    }
}

addEventListener("resize", textSlice, false);