if (addition) {
        // operation = add
    }
    else {
        // operation = subtract
    }

    if (addition) {
        function operation (a, b) { return a + b; }
    }
    else {
        function operation (a, b) { return a - b; }
    }
    console.log(operation(1,2));

Additionally it's much less clear what's going on. Compare it to this:

    var operation;
    if (addition) {
        operation = function add (a, b) { return a + b; };
    }
    else {
        operation = function subtract (a, b) { return a - b; };
    }
    console.log(operation(1, 2));

Personally I prever never to rely on function hoisting and always declare functions using function expressions. It keeps the code style more consistent and it's clearer what is happening in your program.

    var func = _.memoize(function private_non_memoized_func() {
      func(); // "light"
      private_non_memoized_func(); // "heavy"
    });