no-mutating-methods.md

Forbid the use of mutating methods

If you want to program as if your variables are immutable, you should not use mutating methods of core JavaScript objects, such as Array.prototype.push. Due to the lack of static information in JavaScript, it is not possible to know whether a method of an object is indeed the mutating core method or a function with the same name. Therefore, this rule simply reports the use of methods with the following names: copyWithin, pop, push, reverse, shift, sort, splice, unshift.

It will also report the use of Object.defineProperties, Object.defineProperty, Object.setPrototypeOf.

Options

This rule supports the following options:

allowedObjects: Array of names of objects whose methods can be used with no restrictions. This can be useful when using libraries like lodash/fp or Ramda, whose methods will not mutate the arguments. Defaults to an empty array. If set to true, then this rule will not report when assigning to or to a (sub-) property of exports or module.exports. Note that this will not report when reassigning or overwriting previous exports.

You can set the options like this:

"fp/no-mutating-methods": ["error", {
  "allowedObjects": ['_', 'R', 'fp']
}]

Fail

probableArray.copyWithin(a);
probableArray.pop();
probableArray.push(value);
probableArray.reverse();
probableArray.shift();
probableArray.sort();
probableArray.splice(0, 1, value);
probableArray.unshift(value);
probableArray.unwatch();
probableArray.watch(fn);

Object.defineProperties(object, props);
Object.defineProperty(object, 'prop', descriptor);
Object.setPrototypeOf(object, prototype);

Pass

probableArray.foo();
probableArray.concat(otherArray);

Object.keys(object);

/* eslint fp/no-mutating-methods: ["error", {"allowedObjects": ["_"]}] */
var _ = require('lodash/fp');
_.sort(a);