ESLint rules for enforcing safe use of _.extend() / $.extend()
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README.md

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eslint-plugin-extend

ESLint rules for enforcing safe use of Underscore's _.extend(), jQuery's $.extend(), and compatible implementations.

Why Use It

TLDR

If you or someone else on your team sometimes gets confused by the signature of _.extend() / $.extend() and accidentally modifies a source object rather than the destination object, this rule might be for you.

Details

The Underscore library's _.extend() function and jQuery's $.extend() are both used to copy properties of one or more source objects to a specified destination object; they also return the destination object.

Both extend() implementations modify the specified destination object. Consider the following statement:

var newObject = _.extend(objectA, objectB);

In this case, the properties of objectB are copied into objectA. newObject is simply a reference to objectA; both newObject and objectA point to the same, modified, object.

It is often the desire of the developer to create a whole new object into which the properties of source objects are copied. To do so, it is necessary to pass an object literal as the first argument to extend():

var newObject = _.extend({}, objectA, objectB);

Here, newObject now points to a third object: the object literal that we passed in, extended with the properties of objectA and objectB. And objectA is unmolested.

Forgetting to pass in an object literal as the first argument is a common mistake that leads to undesired side effects. Consider the following statement from a dinosaur genome management application built on a fictional MVC framework:

var HerbivorousDinosaurWeAreNotTooAfraidOf = _.extend(Model.Dinosaur, {
    fillGapsWithIffyFrogDNASequence: true
});

Here, the goal was clearly to create a subset of Dinosaur with the specified fillGapsWithIffyFrogDNASequence property defined on it, but we inadvertently modified the Dinosaur itself and are in for a nasty surprise.

The rules added by this plugin allow you to catch this kind of dumb use of _.extend early on, hopefully preventing the need for a lot of future debugging and reducing the number of lawsuits aimed at your live dinosaur theme park.

Getting Started

Documentation for each rule can be found in docs/rules.

Further Reading