jacksonrayhamilton / eslint-plugin-no-var-reassign

Disallow assignment to declared variables

eslint-plugin-no-var-reassign

let and const weren’t really necessary additions to JavaScript; all you need is a linter and var.

var is easier to declare. Function scoping and even redeclaration can both be useful:

if (…) {
  var foo = 1
} else {
  var foo = 2
}
doSomething(foo)

Can’t do that with let or const, since they’re block-scoped.

let and const will throw runtime errors when you try to use their variables before they’ve been declared. However, you could instead rely on ESLint’s no-use-before-define rule to catch such mistakes with var.

One could pretend that a var was a constant, and he could theoretically use a linter to enforce that. Considering the qualities of var, if a linter were to enforce using variables after they were defined and if it were to enforce their constancy, then var could be superior to let or const.

This ESLint plugin provides a rule achieving that: no-var-reassign

Installation

Install ESLint globally or locally:

yarn global add eslint
yarn add -D eslint

Install this plugin globally or locally:

yarn global add eslint-plugin-no-var-reassign
yarn add -D eslint-plugin-no-var-reassign

Configuration

Add “no-var-reassign” to your plugins section, and enable the rule by adding “no-var-reassign/no-var-reassign” to your rules section. Normally this is configured in the .eslintrc config file, like so:

{
  "plugins": [
    "no-var-reassign"
  ],
  "rules": {
    "no-var-reassign/no-var-reassign": "warn"
  }
}

Usage

Now, whenever you try to assign to an already-declared variable, you’ll be warned as if that variable was a constant. Here, ESLint will report an error on line 2:

var foo = 1
foo = 2

Since tail calls eliminate the need for assignment in loops, assignment is never truly necessary in the language. This code (which needed assignment):

for (var i = 0; i < 10; i++) {}

Becomes:

(function loop (i = 0) {
  if (i + 1 < 10) loop(i + 1)
}())

However, if you’re writing code for an environment without tail calls, or if you simply wish to use assignment on some occassions, you can selectively disable the rule. ESLint won’t report an error in the following example:

var foo = 1
if (…) foo = 2 // eslint-disable-line no-var-reassign/no-var-reassign

License

MIT; see the “LICENSE” file.