Using _ (underscore) variable with arrow functions in ES6/Typescript

I came across this construct in an Angular example and I wonder why this is chosen:

_ => console.log('Not using any parameters');

I understand that the variable _ means don't care/not used but since it is the only variable is there any reason to prefer the use of _ over:

() => console.log('Not using any parameters');

Surely this can't be about one character less to type. The () syntax conveys the intent better in my opinion and is also more type specific because otherwise I think the first example should have looked like this:

(_: any) => console.log('Not using any parameters');

In case it matters, this was the context where it was used:

submit(query: string): void {
    this.router.navigate(['search'], { queryParams: { query: query } })
      .then(_ => this.search());
}

Answers:

Answer

The reason why this style can be used (and possibly why it was used here) is that _ is one character shorter than ().

Optional parentheses fall into the same style issue as optional curly brackets. This is a matter of taste and code style for the most part, but verbosity is favoured here because of consistency.

While arrow functions allow a single parameter without parentheses, it is inconsistent with zero, single destructured, single rest and multiple parameters:

let zeroParamFn = () => { ... };
let oneParamFn = param1 => { ... };
let oneParamDestructuredArrFn = ([param1]) => { ... };
let oneParamDestructuredObjFn = ({ param1 }) => { ... };
let twoParamsFn = (param1, param2) => { ... };
let restParamsFn = (...params) => { ... };

Although is declared but never used error was fixed in TypeScript 2.0 for underscored parameters, _ can also trigger unused variable/parameter warning from a linter or IDE. This is a considerable argument against doing this.

_ can be conventionally used for ignored parameters (as the other answer already explained). While this may be considered acceptable, this habit may result in a conflict with _ Underscore/Lodash namespace, also looks confusing when there are multiple ignored parameters. For this reason it is beneficial to have properly named underscored parameters (supported in TS 2.0), also saves time on figuring out function signature and why the parameters are marked as ignored (this defies the purpose of _ parameter as a shortcut):

let fn = (param1, _unusedParam2, param3) => { ... };

For the reasons listed above, I would personally consider _ => { ... } code style a bad tone that should be avoided.

Answer

I guess _ => is just used over () => because _ is common in other languages where it is not allowed to just omit parameters like in JS.

_ is popular in Go and it's also used in Dart to indicate a parameter is ignored and probably others I don't know about.

Answer

It is possisble to distinguish between the two usages, and some frameworks use this to represent different types of callbacks. For example I think nodes express framework uses this to distinguish between types of middleware, for example error handlers use three arguments, while routing uses two.

Such differentiation can look like the example below:

const f1 = () => { } // A function taking no arguments
const f2 = _ => { }  // A function with one argument that doesn't use it

function h(ff) { 
  if(ff.length==0) {
    console.log("No argument function - calling directly");
    ff()
  } else if (ff.length==1) {
    console.log("Single argument function - calling with 1");
    ff(1)
  }
}

h(f1)
h(f2)
Answer

When I wrote the post I was under the impression _ was the only way to create arrow functions without (), which led me to believe using _ could have some minor advantages, but I was wrong. @Halt has confirmed in the comments that it behaves just like other variables, it is not a special language construct.


I want to mention one more thing I realized about these underscore arrow functions while testing myself I didn't find mentioned anywhere. You can use the underscore in the function as a parameter, although this probably isn't intended usage since it's supposed to represent an unused parameter. For clarity, I wouldn't really reccomend using it this way. but it can be useful to know for things like codegolf, challenges where you write the shortest code (it turns out you can just use any character without ()). I could imagine some real use cases where libraries use this and you need to use it even if they didn't intend for that functionality.

Example:

// simple number doubling function
f = _=> {
    _ = _ * 2;
    return _;
}

console.log(f(2)); // returns 4
console.log(f(10)); // returns 20

Tested with Chrome console, Version 76.0.3809.132 (Official Build) (64-bit)

Answer

The () syntax conveys the intent better imho and is also more type specific

Not exactly. () says that the function does not expect any arguments, it doesn't declare any parameters. The function's .length is 0.

If you use _, it explicitly states that the function will be passed one argument, but that you don't care about it. The function's .length will be 1, which might matter in some frameworks.

So from a type perspective, it might be more accurate thing to do (especially when you don't type it with any but, say, _: Event). And as you said, it's one character less to type which is also easier to reach on some keyboards.

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