eslint-plugin-prefer-let

An eslint plugin to encourage semantic of usage of let and const.

Things being basically equal, code should speak to humans first, and computers second. As such, JavaScript codebases should follow the long-standing conventions set forth by both formal symbolic logic and the practice of functional programming.

Usage of the const keyword to bind an intermediate value of a computation places emphasis on the compiler and its role in ensuring that a reference never changes. By contrast using let in the same situation reads, in plain English, the programmer's intent to declare a name value binding.

It is this plugin's opinion that preventing reassignment of let bindings is better accomplished as a linting rule.

const bindings are allowed at the top-level of a module's scope so that it can represent a value that is a true, dependency-free constant such as π, ℯ, etc...

Good:

const PI = 3.14;

function area(radius) {
  let r2 = radius * radius;
  return PI * r2;
}

Bad:

function volume(radius) {
  const a = area(radius);
  return a * radius / 2
}

Installation

You'll first need to install ESLint:

$ npm i eslint --save-dev

Next, install eslint-plugin-prefer-let:

$ npm install eslint-plugin-prefer-let --save-dev

Note: If you installed ESLint globally (using the -g flag) then you must also install eslint-plugin-prefer-let globally.

Usage

Add prefer-let to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
    "plugins": [
        "prefer-let"
    ]
}

Then configure the rules you want to use under the rules section.

{
    "rules": {
        "prefer-let/prefer-let": 2
    }
}

Options

forceUpperCaseConst

When set to true, this option enforces const for top-level UPPER_CASE names (e.g. PI, API_BASE_URL)

{
    "rules": {
        "prefer-let/prefer-let": [2, { "forceUpperCaseConst": true }]
    }
}

This makes the distinction between true constants and regular bindings explicit and machine-enforced.

Good:

const PI = 3.14;
const API_BASE_URL = 'https://example.com';

let config = loadConfig();

Bad:

const config = loadConfig();  // not UPPER_CASE — use let
let PI = 3.14;                // UPPER_CASE — use const

Possible Conflicts

This plugin may conflict with other plugins or configs that set eslint prefer-const. You can configure the rules to avoid this:

{
    "rules": {
        "prefer-let/prefer-let": 2,
        "prefer-const": "off"
    }
}