What is the Underscore for in front of the arrow function (_=>) in the hero.service.ts Angular v5 Tour of Heroes Tutorial?

Question

I have either a typescript or javascript syntax issue. Can someone tell me what _ => this.log... means?

I am used to seeing a name the parameter being passed into the arrow function there.

Does it simply mean 'no parameter'?

Ref: https://angular.io/tutorial/toh-pt6#add-heroserviceupdatehero

    /** PUT: update the hero on the server */
updateHero (hero: Hero): Observable<any> {
  return this.http.put(this.heroesUrl, hero, httpOptions).pipe(
    tap(_ => this.log(`updated hero id=${hero.id}`)),
    catchError(this.handleError<any>('updateHero'))
  );
}

Possible duplicate of [Using \_ (underscore) variable with arrow functions in ES6/Typescript](https://stackoverflow.com/questions/41085189/using-underscore-variable-with-arrow-functions-in-es6-typescript) — Carolus, Jul 25 '19 at 14:33

score 13 · Answer 1 · edited Jul 19 '19 at 13:50

() => {console.log('Hello World')}

_ => {console.log('Hello World')}

Both of the above are just the same if your function doesn't need a parameter.

The underscore _ is just a throwaway variable, meaning it can be any variable name since it will never be used. It's just that they usually use the underscore to say that the function doesn't need a parameter.

I write my functions with no parameters using ()=>, but I've seen a lot of versions using the underscore so it's good to understand both.

score 9 · Accepted Answer · answered Dec 22 '17 at 04:07

9

Its nothing but a notion to name a parameter which isn't going to be used in the function.

Instead, they would have written it like this:

tap(() => this.log(`updated hero id=${hero.id}`)),

If you want to read more, this post is a good start.

answered Dec 22 '17 at 04:07

31piy

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I see now. Any idea why we just could not write: tap( this.log(`updated hero id=${hero.id}`), – OpTech Marketing Dec 22 '17 at 04:16
4

@OpTechMarketing Because `tap` expects a function to call, not a return value. – Andrew Li Dec 22 '17 at 04:18

What is the Underscore for in front of the arrow function (_=>) in the hero.service.ts Angular v5 Tour of Heroes Tutorial?

2 Answers2