no-param-reassign

After price = price * 1.2, the name price no longer holds what the caller passed, and anyone reading further down the function has to notice that. In a function that isn't in strict mode, the assignment also changes arguments[0].

Use a new variable for the new value.

Configure

Off by default. To turn it on:

[lint.rules]
"no-param-reassign" = "error" # or "warn"

Every option, set to its default. The ones that are commented out are unset by default.

[lint.rules]
"no-param-reassign" = ["error", {
  props = false,
  # ignorePropertyModificationsFor = [], # list of strings
  # ignorePropertyModificationsForRegex = [], # list of strings
}]

Examples

Reported:

function total(price, count) {
  price = price * 1.2;
  count++;
  return price * count;
}

Not reported:

function total(price, count) {
  const withTax = price * 1.2;
  return withTax * (count + 1);
}

Options

Option	Default	What it does
props	false	Also report a change to a parameter's property: an assignment, ++, --, delete, or use as the variable of a for...in or for...of loop.
ignorePropertyModificationsFor	[]	With props: names of parameters whose properties can be changed.
ignorePropertyModificationsForRegex	[]	With props: the same, as regular expressions.

By default, changing a property is allowed:

function activate(user) {
  user.active = true;
}

With { props: true }, it's reported, because the caller's object changes:

function activate(user) {
  user.active = true;
  delete user.pending;
}

The two lists make exceptions for parameters that exist to be changed. With { props: true, ignorePropertyModificationsFor: ["res"], ignorePropertyModificationsForRegex: ["^draft"] }:

function notFound(req, res) {
  res.statusCode = 404;
}

function rename(draftUser, name) {
  draftUser.name = name;
}