In Python

I'm sure it's been blogged about a buncha times before but, I couldn't find it, and I had to search too hard to find an example of this. Basically, what I'm trying to do is what Python does in this case, but in JavaScript:


def do_something(arg="notset", **kwargs):
    print(f"arg='{arg.upper()}'")

do_something(arg="peter")
do_something(something="else")
do_something()

In Python, the output of all this is:

arg='PETER'
arg='NOTSET'
arg='NOTSET'

It could also have been implemented in a more verbose way:


def do_something(**kwargs):
    arg = kwargs.get("arg", "notset")
    print(f"arg='{arg.upper()}'")

This more verbose format has the disadvantage that you can't quickly skim it and see and what the default is. That thing (arg = kwargs.get("arg", "notset")) might happen far away deeper in the function, making it hard work to spot the default.

In JavaScript

Here's the equivalent in JavaScript (ES6?):


function doSomething({ arg = "notset", ...kwargs } = {}) {
  return `arg='${arg.toUpperCase()}'`;
}

console.log(doSomething({ arg: "peter" }));
console.log(doSomething({ something: "else" }));
console.log(doSomething());

Same output as in Python:

arg='PETER'
arg='NOTSET'
arg='NOTSET'

Notes

I'm still not convinced I like this syntax. It feels a bit too "hip" and too one-liner'y. But it's also pretty useful.

Mind you, the examples here are contrived because they're so short in terms of the number of arguments used in the function.
A more realistic thing like be a function that lists, upfront, all the possible parameters and for some of them, it wants to point out some defaults. E.g.


function processFolder({
  source,
  destination = "/tmp",
  quiet = false,
  verbose = false
} = {}) {
  console.log({ source, destination, quiet, verbose });
  // outputs
  // { source: '/user', destination: '/tmp', quiet: true, verbose: false }
}

console.log(processFolder({ source: "/user", quiet: true }));

One could maybe argue that arguments that don't have a default are expected to always be supplied so they can be regular arguments like:


function processFolder(source, {
  destination = "/tmp",
  quiet = false,
  verbose = false
} = {}) {
  console.log({ source, destination, quiet, verbose });
  // outputs
  // { source: '/user', destination: '/tmp', quiet: true, verbose: false }
}

console.log(processFolder("/user", { quiet: true }));

But, I quite like keeping all arguments in an object. It makes it easier to write wrapper functions and I find this:


setProfile(
  "My biography here",
  false,
  193.5,
  230,
  ["anders", "bengt"],
  "South Carolina"
);

...harder to read than...


setProfile({
  bio: "My biography here",
  dead: false,
  height: 193.5,
  weight: 230,
  middlenames: ["anders", "bengt"],
  state: "South Carolina"
});

Comments

John

Thanks!

Your email will never ever be published.

Previous:
How depend on a local Node package without npmjs.com January 15, 2020 JavaScript
Next:
How to pad/fill a string by a variable in Python using f-strings January 24, 2020 Python
Related by category:
A Python dict that can report which keys you did not use June 12, 2025 Python
In Python, you have to specify the type and not rely on inference October 10, 2025 Python
Native connection pooling in Django 5 with PostgreSQL June 25, 2025 Python
Combining Django signals with in-memory LRU cache August 9, 2025 Python
Related by keyword:
TypeScript function keyword arguments like Python September 8, 2021 Python, JavaScript