no-mutation.md

Forbid the use of mutating operators

If you want to program as if your variables are immutable, part of the answer is to not mutation by re-assigning to a variable or assigning to a property, and not use update operators like ++ or --.

Options

This rule supports the following options:

• commonjs: 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.

• allowThis: If set to true, then this rule will not report when assigning to or to a (sub-) property of this.

• exceptions: List of objects that describe exceptions to the rule. Each exception should have either or both object and property field set.

You can set the options like this:

"fp/no-mutation": ["error", {
  "commonjs": true,
  "allowThis": true,
  "exceptions": [
    {"object": "foo", "property": "bar"}
  ]
}]

Fail

a = 0;

a += 10;
a -= 10;
a *= 10;
a /= 10;
a %= 10;

--a;
++a;
a--;
a++;

function foo(a) {
  a = a || {};
}

exports = {};
exports.foo = {};
module.exports = {};
module.exports.foo = {};

function foo(a) {
  this.a = a || {};
}

Pass

var a = 0;
let b = 1;
const c = 2;

function foo(a={}) {}

/* eslint fp/no-mutation: ["error", {"commonjs": true}] */
exports = {};
exports.foo = {};
module.exports = {};
module.exports.foo = {};

/* eslint fp/no-mutation: ["error", {"exceptions": [{"property": "propTypes"}]}] */
function Component(props) {
  // ...
}

Component.propTypes = {
  // ...
};

/* eslint fp/no-mutation: ["error", {"allowThis": true}] */
function foo(a) {
  this.a = a || {};
}