flow 0.35.0 "Function cannot be called on possibly undefined value" error after typeof === 'function' check

[nrser]

don't understand how i'm getting this error:

732:       this.gulp.task(gulpName, gulpDeps, function() { return fn(); });
                                                                  ^^^^ function call. Function cannot be called on possibly undefined value
732:       this.gulp.task(gulpName, gulpDeps, function() { return fn(); });
                                                                  ^^ undefined

in the following code (abridged):

setGulpTask(
    name: string,
    depsOrFn: Array<string> | Function,
    fn?: Function,
  ): void {
    // ...
    if (typeof fn === 'function') {
      this.gulp.task(gulpName, gulpDeps, function() { return fn(); });
    } else {
      this.gulp.task(gulpName, gulpDeps);
    }
  }

the usage of fn is inside a test for typeof fn === 'function', which i thought flow understood?

[AugustinLF]

The problem is that function parameters can be reassigned, so flow can't be sure that when fn is called, that it's still valid. You can do something like that to avoid this problem:

setGulpTask(
    name: string,
    depsOrFn: Array<string> | Function,
    fn?: Function,
  ): void {
    // ...
    if (typeof fn === 'function') {
      const callback = fn;
      this.gulp.task(gulpName, gulpDeps, function() { return callback(); });
    } else {
      this.gulp.task(gulpName, gulpDeps);
    }
  }

[nrser]

thanks, that's basically how i ended up solving it.

i see why it is necessary...

function f(fn) {
  const newFn = function() {
    return fn();
  }

  fn = function() {
    return 'redefined';
  }

  return newFn;
}

const arg = function() { return 'defined'; };

f(arg)(); // => 'redefined'

sigh.... such is javascript i guess.