The following table gives an overview of studied performance issues and their root causes. The number in parentheses denotes commit number/s if the pull request contains more performance related optimizations.

| Pull request | Library | Description | Root causes | | --- | --- | --- |--- | --- | | 102 | JQuery | Avoid Array.slice, for the case when argument is negative number | Inefficient API, Inefficient/Redundant copying, Generic API | 248 | JQuery | Give support for more elements to use innerHtml instead of appendChild in html() function | Inefficient API, Special cases | 367 | JQuery |When searching for a given element separate two cases: when element is undefined and when element is string into two conditions, avoid string check when element is undefined | Special cases | 590 | JQuery |Use for loop instead Jquery.each | Inefficient iteration | 725 | JQuery |Avoid unnecessary delegation, instead of .css call .style function that is called inside .css | Inefficient API usage | 7587 | JQuery |Prefer window.JSON at first place if available | Other | 295 1 | JQuery | Not adding arbitrary hooks into hmtl elements| Other | 295 2 | JQuery | Assign value to variable val only when value is not a function, instead of doing it always. Localize val variable to each block | JIT unfrendly | 200 | JQuery | Abstraction of method that clones the DOM element. This avoids calls to Jquery.find method | Inefficient API usage, Inefficient/Redundant copying | 4359 | Angular | Use &1 instead of %2 for even odd checking| Other | 4457 | Angular | Empty() or Html("") JQuery function| Inefficient API usage | 5388 | Angular | To invoke function use f.apply instead of function call with arguments check (args.length)| Inefficient API usage, Special cases | 5457 | Angular | Two calls to CharAt instead of substr| Inefficient API usage | 7012 | Angular | Use Object.keys instead of for in + hop| Inefficient iteration | 7163 | Angular | Caching the child scope class| Repeated execution | 7501 | Angular | Avoid stringification with toJson (JSON.stringify) in case of number| Inefficient API usage, Generic API, Special cases | 7735 | Angular | Native isArray instead of toString call| Inefficient API usage, API reimplementation | 9942 | Angular | Use Array.slice for coping array instead of for loop| Inefficient/Redundant copying | 8898 | Angular | Use for loop instead of forEach (angular method)| Inefficient iteration | 11215 | Angular | Remove unnecessary checks: isArray, isRegExp, isObject, isTypedArray| Inefficient API usage, Repeated execution, Repeated checks| 9369 | Angular | Use textContent property access instead of .text()| Generic API | 8515 (7,8,10) | Angular | Use for loop instead of forEach (angular method)| API reimplementation| 8515 12 | Angular | Dont register DOM listener for $destroy event| Special cases| 8515 31 | Angular | Trim if only string expression| Repeated checks| 8515 39 | Angular | Use objects without prototype and by doing this expensive hasOwnProperty check is avoided| Inefficient API usage| 8515 44 | Angular | Use object without prototype so hasOwnProperty check is not needed to check direct properties | Inefficient API usage| 8515 46 | Angular | Exit method ($broadcast) if no element is listening | Special cases| 9067 | Angular | Use .map built-in function instead of project's one | API reimplementation| 7759 3 | Angular | Replace for in with hop with Object.keys | Inefficient iteration| 7759 4 | Angular | Don't call function that generates keys for angular several time, since only one instance of angular is allowed. Unique id is given once per instance| Other | 7759 7 | Angular | Use classList property of DOM element instead of using getAttribute('class') if classList has browser support | Inefficient API usage | 7759 9 | Angular | document.window has reference to itself, for checking window property of document use this, instead of checking location and document | Inefficient API usage | 7759 11 | Angular | Always use .textContext property access instead of .innerText in some cases | Inefficient API usage | 10351 | Angular | Instead of setter method use getter.assign | Inefficient API usage | 4263 | Ember | Use document.contains to see if element exist in DOM instead of looping DOM tree | Inefficient API usage, API reimplementation | 4329 1 | Ember | Avoid for in, for checking object property use direct access | Repeated checks | 4329 3 | Ember | Use internalized keys for accessing object properties | Other | 4329 4 | Ember | Reduce usage of indexOf, it is considered as an expensive function| Other | 5547 | Ember | Move try catch block in code-critical part | JIT unfriendly | 9991 | Ember | Instead of indexOf(has_this) use lastIndexOf (has_this,0). The prefix has_this is always 'this.' and matches the access of properties like: this.bar | Inefficient API usage 4158 | Ember | When retriving the unique id of the object (method guideFor) first check is it defined. If it is not defined, then generate new one and assign as: obj[prop]=value | Inefficient API usage | 11338 | Ember | Avoid for in loop, use Object.keys | Inefficient iteration | 3174 | Ember | Avoid console.assert, the condition that checks whether the assert should be called is inserted | Inefficient API usage, Repeated checks, Special cases| 3288 | Ember | Use Ember.ArrayPolyfills.indexOf insteadOf Ember.EnumerableUtils.indexOf | Inefficient API usage | 5212 | Ember | Caching computed properties of the object, since they are almost never changed | Inefficient iteration, Repeated execution | 3665 | React | Use undefined===obj[prop] check instead of hasOwnProperty | Inefficient API usage | 934 | React | Simplified generation of node id | Inefficient API usage, Repeated checks | 808 | React | Use textContent instead of innerHtml | Inefficient API usage | 1885 | React | Use RegExp test instead of several indexOf | Inefficient API usage, Generic API | 68 | Underscore | Algorithmic improvement based on user hint (give as input sorted array) | Special cases | 572 | Underscore | Replace Array.concat with Array.push | Inefficient API usage, Inefficient iteration | 39 | Underscore | Replace split.join with replace | Inefficient API usage | 928 | Underscore | Hoist function definition outside loop | Repeated execution, Repeated checks | 1222 | Underscore | Replace for in with .keys and traditional for loop | Inefficient iteration | 1223 | Underscore | Replace for in with .keys and traditional for loop | Inefficient iteration | 1224 | Underscore | Replace for in with .keys and traditional for loop | Inefficient iteration | 1578 | Underscore | Direct function call instead of .call | Inefficient API usage | 1666 | Underscore | When comparing two objects, first compare do they have same number of properties and then is every property equal | Inefficient API usage, Inefficient iteration | 1700 | Underscore | Prefer using .call instead of apply | Inefficient API usage, Inefficient iteration | 1708 | Underscore | Replace functional programming style with for loop | Inefficient API usage, Inefficient iteration | 1860 | Underscore | Explicitly apply an otherwise implicit type conversion to avoid applying it multiple times | Inefficient API usage, Inefficient iteration, Repeated execution | 30 | Underscore.string | Use Array.join instead of concatenation | Inefficient API usage, Inefficient iteration, API reimplementation | 347 1 | Underscore.string | Do implicit string conversion instead of String wrapper in startsWith method | Inefficient API usage | 347 2 | Underscore.string | Dont 'slice' the string, use indexOf and lastIndexOf methods | Inefficient API usage | 2768 | Backbone | Instead of calling push with apply, iterate over set of elements and push each of them separately | Inefficient API usage, Inefficient iteration | 2858 | Backbone | In the function that removes all callbacks for all events, the statement events={} is replaced by events=void 0 | Other | 1766 | Backbone | Avoid unnecessary object creation | Inefficient/Redundant copying | 1097 | Backbone | Caching regular expression | Repeated execution | 136a | EJS | Replaced buf.push() and buf.join() with string concatentation | Inefficient API usage | 136b | EJS | Replaced string.substr(i, 1) with string[i] | Inefficient API usage, Inefficient/Redundant copying | 136c | EJS | Hoisted str[i] out of loop| Repeated execution | 1625 1 | Less | Cache array of functions for faster lookup | Repeated execution | 1831 | Less | Caching matching elements | Repeated execution | 317 | Moment | Cache functions that are likely to be reused on the same object with the same input | Inefficient API usage, Repeated execution | 1875 | Moment | Assuming that object has finite number of properties, instead of interating through list of properties and checking is it defined in object, try to check them in if conditionals | Inefficient API usage, Inefficient/Redundant copying | 1885 | Moment | Adding check in the case of null input parameter and adapting the method's behaviour for this case | Generic API, Inefficient/Redundant copying, Special cases | 8 | Node-lru-cache | Avoid Object.keys(obj)[i]; instead use for in loop. | Inefficient API usage, Inefficient iteration, Inefficient/Redundant copying | 8 | Node-lru-cache | Avoid Object.keys(obj)[i]; instead use for in loop. | Inefficient API usage, Inefficient iteration, Inefficient/Redundant copying | 388 (2) | Cheerio | Avoid uneccessary function call in conditional | Repeated checks | 388 (5) | Cheerio | String concat instead of join | Inefficient API usage | 386 | Cheerio | Avoid unnecessary computation | Generic API | 385 | Cheerio | Check more on what proposed function do, but the idea is to switch to another, cheaper function call | Inefficient API usage | 369 (5)| Cheerio | Inline object lookup | Inefficient API usage, Inefficient/Redundant copying | 369 (8)| Cheerio | Avoid recomputing value that will be reused | Repeated execution | 27a | Chalk | Don't concatenate the arguments into string with slice.join if there is only one argument. In latter case use explicit string concatenation | Inefficient API usage, Generic API, Special cases | 27b | Chalk | Precomputed style function. Merge multiple return statements to one | Other | 28 | Chalk | Replace Array.prototype.reduce() with explicit for loop | Inefficient iteration | 701 | Mocha | Use instanceof where supported instead of toString.call(err) === "[object Error]" | Inefficient API usage | 763 | Mocha | The first commit is using Object.keys where possible instead of checking each property of the object | Inefficient API usage, API reimplementation, Inefficient/Redundant copying | 403 | Request | Computation from the if is moved to a global variable, so it is done only once | Repeated execution, Repeated checks | 1165 | Request | Remove everything from init() method that is not related to request initialization | Repeated execution | 573 | Socket.io | Optimized switch, the most common cases are checked first | Other | 689 | Socket.io | Combine two substring calls into one | Inefficient API usage, Inefficient/Redundant copying |