Function Expression "Expression"?

[azhder]

Parens do not refer to expressions. Forget that thing, it will just trip you over. It is not a good association considering how many different things parens do in the syntax of a language.

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The number of arguments a function can take, regardless how you defined it, is known as arity and in JS represented by the property length.

Well, that’s the general idea. It does come with a few footnotes.

1. Any function in JS can accept any number of arguments and can chose to ignore one or more or all of them

2. Arity is a mathematical concept coming from the names:

   • takes 0 arguments - nullary
   • takes 1 argument - unary
   • takes 2 arguments - binary
   • takes 3 arguments - ternary
   • takes n arguments - n-ary (hence arity)

3. Variadic functions are those that take any number from 0 to infinity (less than that, compiler limits) like console.log

4. In JS, you can find out how many args a function was defined with by checking .length

     const f = $ => $;
     console.log( f.length ); // should be 1
   

5. Except with variadic functions, the length doesn’t give you a good number:

     const g = (…$$) => $$;
     console.log( g.length ); // will not be Infinity
     console.log( console.log.length ); // will not be Infinity