Will static javascript function using only constants be compiled to constant

Question

Will the following javascript

var StaticTest_A = (function () {
  function StaticTest_A() {
  }
  StaticTest_A.GetConstant = function () {
    return StaticTest_B.MyNumber + StaticTest_B.MyNumber;
  };
  return StaticTest_A;
})();
var StaticTest_B = (function () {
  function StaticTest_B() {
}
StaticTest_B.MyNumber = 5;
  return StaticTest_B;
})();

generated from this TypeScript

class StaticTest_A
{
   static GetConstant():number
   {
       return StaticTest_B.MyNumber + StaticTest_B.MyNumber;
   }
}

class StaticTest_B
{
   static MyNumber = 5;
}

be compiled so StaticTest_A.GetConstant() returns a constant or will the function be calculated on every call?

Nothing in JavaScript can guarantee that `StaticTest_B.MyNumber` won't be changed, so the function has to be re-evaluated. — Pointy, Apr 29 '16 at 20:15
Just add an `alert` or something and you will see the code runs again for each call, it's not cached. — Oriol, Apr 29 '16 at 20:16

score 3 · Accepted Answer · edited May 23 '17 at 12:23

No, that expression will run each time and it should because StaticTests_B.MyNumber could change in the meantime, as Pointy pointed out. Additionally, TypeScript doesn't change the code you write to improve the performance—that's actually a non-goal of the compiler.

If you only want the calculation to happen once for performance reasons then you will have to come up with your own solution to that. For example, you could create a reusable decorator that caches the value the first time it's run and then returns the cached value afterwards. You could then use that decorator anywhere you like:

function methodMemoize(target: Function | Object, key: string, descriptor: TypedPropertyDescriptor<() => any>) {
    const originalValue = descriptor.value;
    let hasRun = false;
    let returnedValue: any;

    descriptor.value = function(...args: any[]) {
        if (!hasRun) {
            returnedValue = originalValue.apply(this, args);
            hasRun = true;
        }

        return returnedValue;
    };
}

class StaticTest_A
{
    @methodMemoize
    static GetConstant()
    {
        console.log("run only once");
        return StaticTest_B.MyNumber + StaticTest_B.MyNumber;
    }
}

class StaticTest_B
{
   static MyNumber = 5;
}

StaticTest_A.GetConstant(); // "run only once", returns 10
StaticTest_A.GetConstant(); // returns cached 10

Thanks for the alternative solution – noontz Apr 30 '16 at 12:29 — noontz, Apr 30 '16 at 12:29

Will static javascript function using only constants be compiled to constant

1 Answers1