no-extraneous-iteratee-args.md

No extraneous parameters in iteratees

In lodash/fp, iteratees (the functions passed to methods like _.map, _.reduce or _.forEach) have their arguments capped, meaning that instead of being called with iteratee(value, index|key), they are called with iteratee(value).

That is done to avoid problems with iteratees that can accept multiple arguments. Try out ['1', '2', '3'].forEach(parseInt); and see if the result is the same as what you expected it to be.

This rule will report an error when you are passing an iteratee to a Lodash method, that accepts more arguments than that will be passed to it. Note that as of this moment, this rule will not report anything when being passed a function that is declared somewhere else in the code, see passing example below.

Rule Details

Fail

const array = [1, 2, 3];
_.map(function iteratee(value, index) { // index is ignored
  return value * index;
}, array);

_.map((value, index) => value * index); // index is ignored

_.reduce(function iteratee(result, value, index) { // index is ignored
  return result + (value * index);
}, array);

Pass

const array = [1, 2, 3];
_.map(function iteratee(value) {
  return value * 2;
}, array);

_.map((value) => value * 2);

_.reduce(function iteratee(result, value) {
  return result + value;
}, array);

// The following is not yet reported, but will lead to problems
const iteratee = (value, index) => value * index;
_.map(iteratee, array);