Prefer literal function declaration to function expression for performance reasons

[iirelu]

As shown by this jsperf here, function declaration is anywhere between twice as fast and hundreds of times faster than assigning an anonymous function to a variable. Searching through the code reveals that function expressions are used predominantly (~314 times) whilst function declaration is only used 10 times.

[gorhill]

http://jsperf.com/function-declarations-vs-function-expressions/5

[iirelu]

I agree that it's a micro-optimisation, but it also works to improve general quality of code. However, this is an opinionated style point.

[gorhill]

There is not even a micro-optimisation. The original benchmark is completely flawed. Here is the generated code by the benchmark:

while (i14100320934181105--) {
    function X() {
        return "hello";
    }
    function Y() {
        return X();
    }
    function Z() {
        return Y();
    }
    Z();
}

Which is equivalent to:

function X() {
    return "hello";
}
function Y() {
    return X();
}
function Z() {
    return Y();
}
while (i14100320934181105--) {
    Z();
}

Because of hoisting, the functions are defined once by the benchmark. Function expressions do not benefit from hoisting. Nowhere in uBlock are functions defined in a loop as above, so there is no benefit from converting function declarations to function expressions.

In general I have come to prefer function expressions, because there is no surprise through hoisting: where the declaration appear is where they start to exist.