no-param-reassign: function type exceptions

What version of ESLint are you using?

2.11.1

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

I’m generally OK with airbnb-base concerning this rule:

'no-param-reassign': [2, { 'props': true }],

What did you do?..

However, it would be useful to have exceptions for the whole function types. In my code I’ve bumped into these cases in which eslint was too strict as for me.

1. Proxy traps.

The target argument for many traps is intended to be modified. See as example: Validation

eslint output: error Assignment to property of function parameter 'obj' no-param-reassign

1. Node.js stream listeners.

Expanded example from Read File Stream Line-by-Line:

const readline = require('readline');
const fs = require('fs');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt')
});

let lineNumber = 0;
rl.on('line', line => {
  if (++lineNumber === 1) line = line.replace(bomRegExp, '');

  line = line.trim();

  if (line) {
    // Do something with the line data.
  }
});

eslint output: Assignment to function parameter 'line' no-param-reassign

In such cases creation an intermediate variable could be overcomplication. May be some additions to the rule like proxyTraps: false, eventListeners: false would be helpful.

[dacz]

reduce is a function that would be nice to allow modifying accumulator (with the rule on).

[ilyavolodin]

@vsemozhetbyt It seems that what you want goes completely against the idea behind no-param-reassign rule. Why do you want to keep it on in the codebase that requires overriding parameters? Also, couldn’t you assign line to a temporary variable inside your event handler and modify that variable instead?