How to check if function exists?

Question

I am updating some old Javascript to Typescript. In Javascript you can do the following [ref]:

if (typeof functionName === "function") { 
    // safe to use the function
    functionName();
}

In Typescript this gives a syntax error "Cannot find name 'updateRadarCharts'"

I can resolve the issue with a declare statement

declare var functionName: Function;

However this doesn't feel like a clean solution because it is possible this won't be declared (hence the check). Is there a cleaner way to do this in TS ?

https://stackoverflow.com/questions/1042138/how-to-check-if-function-exists-in-javascript — Nicklas Pouey-Winger, Feb 24 '19 at 22:37

score 4 · Accepted Answer · answered Feb 24 '19 at 22:39

4

You could declare the function as:

declare var functionName: Function | undefined;

answered Feb 24 '19 at 22:39

Evert

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score 1 · Answer 2 · answered Feb 25 '19 at 05:32

For global augmentation (which seems to be what you are trying to achieve), a user defined type guard usually works well:

interface AugmentedGlobal {
  something: SomeType;
}

function isAugmented(obj: any): obj is AugmentedGlobal {
  return 'something' in obj;
}

if (isAugmented(global/**or window*/)) {
  const myStuff = global.something;
}

How to check if function exists?

2 Answers2