Thoughts on "Never mutate parameters"

1- This rule’s “Do not reassign parameters” part should be split into another rule, because reassigning a parameter isn’t actually mutation.

Plus it sounds like no-param-reassign rule also covers mutation, but it doesn’t. It would be nice to easily see what gets covered by ESLint and what doesn’t.

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2- Why shouldn’t we reassign parameters?

Overwriting parameters can lead to unexpected behavior, especially when accessing the arguments object.

Accessing arguments is already discouraged. Should we really care about breaking something we aren’t supposed to use? What are other unexpected behaviors can this cause?

[ljharb]

Reassigning parameters deoptimizes in many engines, specifically v8 - it’s a horrible idea to do it. Variables are free, and creating new ones rather than reusing old ones makes code much clearer.

(In addition, nothing in JS is passed by reference, everything is passed by value, where objects are a form of “reference value” - this is a good read on the subject)

A PR would be welcome if there’s verbage that could be cleared up.

[AlicanC]

Reassigning parameters doesn’t do that. Mutating parameters do that.

const myObj = {
  adele: 'hello',
};

function myFunc(myParam) {
  myParam = {};

  myParam.adele = 'rolling in the deep';
}

myFunc(myObj);

console.log(myObj); // { adele: 'hello' }

Mutating parameters is wrong, I have no objection to that rule.