README

eslint-plugin-better-mutation

ESLint rules so you and your team use immutable values when you should, and permits mutation when it's safe.

Preventing unsafe mutation of shared variables prevents a huge class of bugs from ever appearing.

The plugin's goal is to prevent modification of shared variables, such as function parameters or globals whether via assignment, operators, functions or methods.

Locally declared variables are permitted to use mutation, because in most circumstances this is safe.

See WHY.

What's safe?

Block scoped (let) or function scoped variables (var) can be reassigned safely. Even objects or arrays marked with const can have nested properties changed within the block scope they belong to. The same rules apply for mutating functions (e.g. Object.assign() and mutating methods [].push()

function foo() {
  let i = 1; 
  i = 2;
  
  const o = { a: 0 };
  o.a += 1;
}

module.exports = { foo }; // by default commonjs exports are configured to be safe

Also modification of parameters within reducer functions/methods is safe. The parameters of a reducer a safe because the loop that the reducer executes limits the scope of the accumulator to the reducer function.

function sum(numbers) {
  return numbers.reduce((acc, val) => {
    acc += val;  // this is safe!
    return acc;    
  }, 0);
}

What's not safe?

Mutating variables of shared variables is unsafe. This means reassignment outside of the scope they are declared within whether its because they are globals, function parameters, or closed over variables can lead to undefined behaviour that is hard to debug and can cause race conditions. Instead of mutating always return a new value.

g = 2 // global reassignment is unsafe

function foo(i) {
   i += 1 // don't resassign function parameters
}

let a = 1; 
function bar() { 
   a = 2; // don't reassign closed over vars
}

Install

$ npm install --save-dev eslint eslint-plugin-better-mutation

Usage

Configure it in .eslintrc.

{
  "name": "my-awesome-project",
  "eslintConfig": {
    "env": {
      "es6": true
    },
    "plugins": [
      "better-mutation"
    ],
    "rules": {
      "better-mutation/no-mutating-functions": "error",
      "better-mutation/no-mutating-methods": "error",
      "better-mutation/no-mutation": "error",
    }
  }
}

Rules

no-mutation - Forbid the use of mutating operators on non-local variables.
no-mutating-methods - Forbid the use of mutating methods on non-local variables such as push or _.remove.
no-mutating-functions - Forbid the use of Object.assign() and lodash mutation methods with a non-local variable as first argument.

Recommended configuration

This plugin exports a recommended configuration that enforces good practices.

To enable this configuration, use the extends property in your .eslintrc.

{
  "name": "my-awesome-project",
  "eslintConfig": {
    "plugins": [
      "better-mutation"
    ],
    "extends": "plugin:better-mutation/recommended"
  }
}

See ESLint documentation for more information about extending configuration files.

Thanks to Jeroen Engels for providing the basis of this plugin. Checkout https://github.com/jfmengels/eslint-plugin-fp for a strict functional programming approach.

@bdwain/eslint-plugin-better-mutation

Usage no npm install needed!