Variable in a function still accessible after function call

Question

This is an interview question/answer I found online (link).
Original Question: How could you cache execution of any function?

Example Answer:

function cacheFn(fn) {
    var cache = {};
    return function wrapper(arg){
        if (cache[arg]){
           console.log('used cache!!');
           return cache[arg];
        }
        else{
           cache[arg] = fn(arg);
            return cache[arg];
        }
    }
}

My initial idea was that, cache will be removed after cacheFn is first called so no caching will be done. But when I tested it as below...

function func(n) {
  return n+1;
}

const cachef1 = cacheFn(func);
console.log(cachef1(1)); 
console.log(cachef1(1)); // 'used cache!!'

I'm not understanding how var cache = {}; survived after the first cachef1(1); call. It's as if when assigning cacheFn(func1) to cachef1, the whole block inside cacheFn were saved. I also found this does not do caching:

cacheFn(func)(1);
cacheFn(func)(1); // no caching when there was no assignment

What concept am I missing or are confused about here?