Make jQuery Optional

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+- Start Date: 2015-07-06
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Summary
+
+Make jQuery optional.
+
+# Motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+I use jQuery heavily and like it! But decoupling it from Ember have some benefits: it's not always needed,
+some people think payload size is important (I don't) and it may have minor performance contributions.
+
+# Detailed design
+
+I've tried to track down the places where jQuery is used and its implications.
+There is some primary usage (around events) as well as some auxiliary usage, where it should be fairly easy
+to replace it (I hope).
+
+**Primary use** *(will require insight from people with good knowledge of browser difference)*
+
+- [ ] For event dispatching [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-views/lib/system/event_dispatcher.js)
+- [ ] *Have I missed something?*
+
+**Auxiliary use** *(these should be easier to replace)*
+
+- [ ] basic DOM-operations for bootstrap [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-htmlbars/lib/system/bootstrap.js)
+- [ ] Firefox 11 and outerHTML[1]. Is this still needed? Firefox is on v38 now. [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-views/lib/compat/render_buffer.js#L577)
+- [ ] #findElementInParentElement [(code)](https://github.com/emberjs/ember.js/blob/49d58530527c9025b4bafe4f937b0c78cfe3818e/packages/ember-views/lib/views/view.js#L1133)
+- [ ] #replaceIn [(code)](https://github.com/emberjs/ember.js/blob/49d58530527c9025b4bafe4f937b0c78cfe3818e/packages/ember-views/lib/views/view.js#L1038)
+- [ ] #appendTo [(code)](https://github.com/emberjs/ember.js/blob/49d58530527c9025b4bafe4f937b0c78cfe3818e/packages/ember-views/lib/views/view.js#L961)
+- [ ] event listening for location changes and a DOM-lookup [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-routing/lib/location/history_location.js)
+- [ ] getElementById lookup [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-views/lib/views/states/has_element.js#L25)
+
+**Library and access** *(these should remain even if the library is optional, with a friendly message if jQuery is not present)*
+
+- [ ] For `Ember.$`. [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-views/lib/main.js#L55)
+- [ ] Core library registration [(code)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-application/lib/system/application.js#L1141)
+- [ ] The view wrapper `this.$()` [(code 1)](https://github.com/emberjs/ember.js/blob/49d58530527c9025b4bafe4f937b0c78cfe3818e/packages/ember-views/lib/views/view.js#L918) and [(code 2)](https://github.com/emberjs/ember.js/blob/680f997ed0958c420abdcd0b1673111aee26afe7/packages/ember-views/lib/views/states/has_element.js#L16)
+
+Please add to the list if I've missed something.
+
+Since jQuery is good at patching over browser-difference, care must be taken to understand where it's being
+relied on for this.
+
+***
+
+I'll not be able to make all the changes myself, so I'll depend on others helping out.
+
+Instead of tearing it all out, a good idea would probably be to remove some of the smaller jQuery usage first, with
+separate PRs.
+
+Then use a feature-flag to toggle the larger stuff, e.g. event handling with/without jQuery,
+since these will need much more testing to find bugs or issues.
+
+# Drawbacks
+
+- It works today, why bother.
+- jQuery patches over important browser differences.
+
+# Alternatives
+
+Wait 1-2 years, then axe jQuery.
+
+# Unresolved questions
+
+- How would the core team like to handle the 'make it optional' switch? May affect the build process,
+library requirements etc.
+- The FF11 requirement, what is the minimum supported version of FF? Is it fine to drop? (FF is on ~38 now)
+
+# References
+
+[1] https://developer.mozilla.org/en-US/docs/Web/API/Element/outerHTML