How to handle eslint no-param-reassign rule in Array.prototype.reduce() functions

I've recently added the eslint rule no-param-reassign.

However, when I use reduce to build out an object (empty object as initialValue), I find myself needing to modify the accumulator (first arg of callback function) on each callback iteration, which causes a no-param-reassign linter complaint (as one would expect it would).

const newObject = ['a', 'b', 'c'].reduce((result, item, index) => {
  result[item] = index; // <-- causes the no-param-reassign complaint
  return result;
}, {});

Is there a better way to build out an object with reduce that doesn't modify the accumulator argument?

Or should I simply disable the linting rule for that line in my reduce callback functions?

Solution 1:

One solution would be to leverage the object spread operator

const newObject = ['a', 'b', 'c'].reduce((result, item, index) => ({
  ...result,
  [item]: index, 
}), {});

Performance Warning!! This is a non-performant (O(n^2) solution that should be avoided.

Solution 2:

I just wrap the reduce functions in a lint rule disable block, ie:

/* eslint-disable no-param-reassign */
const newObject = ['a', 'b', 'c'].reduce((result, item, index) => {
  result[item] = index;
  return result;
}, {});
/* eslint-enable no-param-reassign */

Solution 3:

Well, you could do (result, item) => Object.assign({}, result, {[item]: whatever}) to create a new object on every iteration :-)

If you want to trick the linter, you could use => Object.assign(result, {[item]: whatever}) (which does the same as your current code but without an explicit assignment), but yeah I guess you should simply disable that rule.