2.0.0 • Public • Published

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A set of ESLint rules that help catch undeclared references that ESLint's built-in no-undef rule doesn't find because they are extending other objects
(AKA "piggybacking" 😛)


This ESLint plugin is useful for migrating a large legacy project that relies on global scope "pollution" into one where all module dependencies are properly declared (via AMD, CommonJS, ES6 Modules, etc.) and automatic module-bundling is possible.
For the most part ESLint's no-undef will get you most of the way there, but each of these custom rules handle a certain edge-case that it does not.


Consider the result of linting this code with ESLint when just the no-undef rule is in-use:

/*eslint-env browser*/; //`Foo` is not defined in this script. `no-undef` will catch this
window.location.reload(); //`window` is not defined in this script but `no-undef` will not throw an error because `window` has been white-listed as a valid global
window.Foo.baz(); //This will not throw an error despite `Foo` not being defined anywhere and is not a valid property of `window`

In the case above browser is set as the environment because you want to access window.location which is valid because it exists in any browser. But once you do this anyone can assign and access other properties on window without triggering a lint violation, which is why no error is thrown for window.Foo.baz().

The no-restricted-global-extend rule can identify cases where you're using an object which references the global scope (e.g. window in front-end code), and it will alert you if the property you're accessing is not available in the global scope in that environment.

With both no-undef and no-restricted-global-extend in-use:

/*eslint-env browser*/; //`Foo` is not defined in this script. `no-undef` will catch this
window.location.reload(); //`window` is not defined in this script but `no-undef` will not throw an error because `window` has been white-listed as a valid global
window.Foo.baz(); //This will now throw an error


This rule helps catch cases where you're using jQuery plugins (e.g. $.cookie, $.query).
jQuery plugins are somewhat of an anti-pattern when it comes to properly declaring your module dependencies because they don't export anything, but rather extend the jQuery object itself.
Generally speaking, if you're refactoring old code that uses a jQuery plugin, there's probably a modern library available that provides the same functionality without relying on or extending jQuery.

With no-jquery-extend in-use:

import $ from 'jquery';
$.ajax( ... ); //This is fine.
$.when( ... ); //This is fine.
$.cookie( ... ); //This will now trigger an ESLint error

Note: This rule will only catch jQuery plugins that extend the jQuery object. It cannot catch jQuery plugins that extend jQuery elements (via jQuery.fn).


You'll first need to install ESLint:

$ npm i eslint --save-dev

Next, install eslint-plugin-piggyback:

$ npm install eslint-plugin-piggyback --save-dev

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


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

    "plugins": [

Add the no-restricted-global-extend rule

Configure the 'no-restricted-global-extend' under the rules section in your .eslintrc configuration file. You must specify which identifiers reference the global scope in your environment (e.g. 'window').

    "rules": {
        "piggyback/no-restricted-global-extend": [2, "window"]

Add the no-jquery-extend rule

Configure the 'no-jquery-extend' under the rules section in your .eslintrc configuration file. You can specify which identifiers reference the jQuery library in your code (default: $ & jQuery).

    "rules": {
        "piggyback/no-jquery-extend": [2, "$", "jQuery", "myJQuery"]


2.0.0 - Switch to ESLint's new rule format. This is a breaking change because as of this ESLint version >= 2.13.0 is required. 1.0.0 - Add the no-jquery-extend rule. This is a breaking change because as of this Node.js version >= 4 is required.

Running tests

Run mocha tests with npm test

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npm i eslint-plugin-piggyback

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