tc39 · Kingwl · Nov 11, 2020 · Nov 11, 2020 · Nov 11, 2020
diff --git a/README.md b/README.md
@@ -1,57 +1,75 @@
 # [proposal-array-find-right](https://kingwl.github.io/proposal-array-find-right/index.html)
 
-Proposal for a `.findRight()` method on array.
+Proposal for `.findRight()` and `.findIndex` methods on array.
 
 ## Motivation
 
 Find something from array is a very common pattern in development. 
 
 Now we have `Array.prototype.indexOf`, `Array.prototype.lastIndexOf` to find index of some `value` in the array.
 
-And we also has `Array.prototype.find` to find some element who in the array in out customized way. 
+And we also has `Array.prototype.find`, `Array.prototype.findIndex` to find some element or index who in the array in out customized way. 
 
 But There’s not a way to allow us find something `from the end to the start ` of array in customized way. 
 
 `[].reverse().find()` is work. But there’s two issues:
 
-1. unnecessary reverse.
-2. `Array.prototype.reverse` is not immutable.
+1. **unnecessary reverse.**
+2. **`Array.prototype.reverse` is not immutable.**
 
-You have to write the `findRight` in your codebase or `[...[]].reverse().find()`. 
+You have to write `[...[]].reverse().find()`. 
 
-As the result the third issue:
+As the result, the third issue:
 
-3. unnecessary spread
+3. **unnecessary spread**
 
-So, perhaps we need `Array.prototype.findRight`.
+For `.findIndex()`,  you must have much additional operations (re-calculate index and handle the `-1` ) for the result of `[...arr].reverse().findIndex()`.
+
+As the result, the fourth issue:
+
+4. **complex index calculate**
+
+So, perhaps we need `Array.prototype.findRight` and `Array.prototype.findIndexRight`.
 
 ## Core features
 
-Add `Array.prototype.findRight`. 
+Add `Array.prototype.findRight` and `Array.prototype.findIndexRight`. 
 
-And we could use that like the [(Array.prototype.find)](https://www.ecma-international.org/ecma-262/11.0/index.html#sec-array.prototype.find) but from the end to be start.
+And we could use that like the [Array.prototype.find](https://www.ecma-international.org/ecma-262/11.0/index.html#sec-array.prototype.find) and [Array.prototype.findIndex](https://www.ecma-international.org/ecma-262/11.0/index.html#sec-array.prototype.findindex) but from the end to be start.
 
 eg:
 
 ```javascript
 const array = [{ value: 1 }, { value: 2 }, { value: 3 }, { value: 4 }];
 
 array.find(n => n.value % 2 === 1); // { value: 1 }
+array.findIndex(n => n.value % 2 === 1); // 0
 
-// Before the proposal
+// ======== Before the proposal =========== 
+
+// find
 [...array].reverse().find(n => n.value % 2 === 1); // { value: 3 }
 
-// In the proposal
+// findIndex
+array.length - 1 - [...array].reverse().findIndex(n => n.value % 2 === 1); // 2
+array.length - 1 - [...array].reverse().findIndex(n => n.value === 42); // should be -1, but 4
+
+// ======== In the proposal =========== 
+// find
 array.findRight(n => n.value % 2 === 1); // { value: 3 }
 
+// findIndex
+array.findIndexRight(n => n.value % 2 === 1); // 2
+array.findIndexRight(n => n.value === 42); // -1
 
 ```
 
 ## Related
 
 - [Array.prototype.reduceRight](https://www.ecma-international.org/ecma-262/11.0/index.html#sec-array.prototype.reduceright)
 - [lodash.findLast](https://lodash.com/docs/4.17.15#findLast)
+- [lodash.findLastIndex](https://lodash.com/docs/4.17.15#findLastIndex)
 - [ramda.findLast](https://ramdajs.com/docs/#findLast)
-
+- [ramda.findLastIndex](https://ramdajs.com/docs/#findLastIndex)
 - [@extra-array/find-right](https://www.npmjs.com/package/@extra-array/find-right)
 
diff --git a/index.html b/index.html
@@ -1845,12 +1845,17 @@
     display: none; 
   }
 }
-</style></head><body><div id="menu-toggle">☰</div><div id="menu-spacer"></div><div id="menu"><div id="menu-search"><input type="text" id="menu-search-box" placeholder="Search..."><div id="menu-search-results" class="inactive"></div></div><div id="menu-pins"><div class="menu-pane-header">Pins</div><ul id="menu-pins-list"></ul></div><div class="menu-pane-header">Table of Contents</div><div id="menu-toc"><ol class="toc"><li><span class="item-toggle-none"></span><a href="#sec-demo-clause" title="Array.prototype.findRight"><span class="secnum">1</span> Array.prototype.findRight</a></li><li><span class="item-toggle-none"></span><a href="#sec-copyright-and-software-license" title="Copyright &amp; Software License"><span class="secnum">A</span> Copyright &amp; Software License</a></li></ol></div></div><div id="spec-container"><h1 class="version first">Stage 0 Draft / November 10, 2020</h1><h1 class="title">Proposal Array.prototype.findRight</h1>
+</style></head><body><div id="menu-toggle">☰</div><div id="menu-spacer"></div><div id="menu"><div id="menu-search"><input type="text" id="menu-search-box" placeholder="Search..."><div id="menu-search-results" class="inactive"></div></div><div id="menu-pins"><div class="menu-pane-header">Pins</div><ul id="menu-pins-list"></ul></div><div class="menu-pane-header">Table of Contents</div><div id="menu-toc"><ol class="toc"><li><span class="item-toggle-none"></span><a href="#sec-demo-clause" title="Array.prototype.findRight"><span class="secnum">1</span> Array.prototype.findRight</a></li><li><span class="item-toggle-none"></span><a href="#sec-copyright-and-software-license" title="Copyright &amp; Software License"><span class="secnum">A</span> Copyright &amp; Software License</a></li></ol></div></div><div id="spec-container"><h1 class="version first">Stage 0 Draft / November 11, 2020</h1><h1 class="title">Proposal Array.prototype.findRight</h1>
 
 <emu-clause id="sec-demo-clause">
   <h1><span class="secnum">1</span> Array.prototype.findRight</h1>
-  <p>The find method is called with one or two arguments, predicate and thisArg.</p>
+  <p>The findRight method is called with one or two arguments, predicate and thisArg.</p>
+  <p>When the findRight method is called, the following steps are taken:</p>
   <emu-alg><ol><li>Let O be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(this value).</li><li>Let idx be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(O).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(predicate) is false, throw a TypeError exception.</li><li>Repeat, while idx &gt;= 0<ol><li>Let Pi be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(idx).</li><li>Let iValue be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(O, Pi).</li><li>Let testResult be !&nbsp;<emu-xref aoid="ToBoolean" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-toboolean">ToBoolean</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(predicate, thisArg, « iValue, idx, O »)).</li><li>If testResult is true, return iValue.</li><li>Set idx to idx - 1</li></ol></li><li>Return undefined</li></ol></emu-alg>
+  <h1>Array.prototype.findIndexRight</h1>
+  <p>The findIndexRight method is called with one or two arguments, predicate and thisArg.</p>
+  <p>When the findIndexRight method is called with one or two arguments, the following steps are taken:</p>
+  <emu-alg><ol><li>Let O be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(this value).</li><li>Let idx be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(O).</li><li>If <emu-xref aoid="IsCallable" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(predicate) is false, throw a TypeError exception.</li><li>Repeat, while idx &gt;= 0<ol><li>Let Pi be !&nbsp;<emu-xref aoid="ToString" id="_ref_10"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(idx).</li><li>Let iValue be ?&nbsp;<emu-xref aoid="Get" id="_ref_11"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(O, Pi).</li><li>Let testResult be !&nbsp;<emu-xref aoid="ToBoolean" id="_ref_12"><a href="https://tc39.es/ecma262/#sec-toboolean">ToBoolean</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_13"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(predicate, thisArg, « iValue, idx, O »)).</li><li>If testResult is true, return idx.</li><li>Set idx to idx - 1</li></ol></li><li>Return -1</li></ol></emu-alg>
 </emu-clause><emu-annex id="sec-copyright-and-software-license">
       <h1><span class="secnum">A</span> Copyright &amp; Software License</h1>
 

diff --git a/spec.emu b/spec.emu
@@ -11,7 +11,8 @@ contributors: Wenlu Wang
 
 <emu-clause id="sec-demo-clause">
   <h1>Array.prototype.findRight</h1>
-  <p>The find method is called with one or two arguments, predicate and thisArg.</p>
+  <p>The findRight method is called with one or two arguments, predicate and thisArg.</p>
+  <p>When the findRight method is called, the following steps are taken:</p>
   <emu-alg>
     1. Let O be ? ToObject(this value).
     2. Let idx be ? LengthOfArrayLike(O).
@@ -24,4 +25,19 @@ contributors: Wenlu Wang
       5. Set idx to idx - 1
     5. Return undefined
   </emu-alg>
+  <h1>Array.prototype.findIndexRight</h1>
+  <p>The findIndexRight method is called with one or two arguments, predicate and thisArg.</p>
+  <p>When the findIndexRight method is called with one or two arguments, the following steps are taken:</p>
+  <emu-alg>
+    1. Let O be ? ToObject(this value).
+    2. Let idx be ? LengthOfArrayLike(O).
+    3. If IsCallable(predicate) is false, throw a TypeError exception.
+    4. Repeat, while idx >= 0
+      1. Let Pi be ! ToString(idx).
+      2. Let iValue be ? Get(O, Pi).
+      3. Let testResult be ! ToBoolean(? Call(predicate, thisArg, « iValue, idx, O »)).
+      4. If testResult is true, return idx.
+      5. Set idx to idx - 1
+    5. Return -1
+  </emu-alg>
 </emu-clause>