Editorial: Introduce StringLastIndexOf (#3290)

spec.html

@@ -1168,7 +1168,7 @@ <h1>
             _string_: a String,
             _searchValue_: a String,
             _fromIndex_: a non-negative integer,
-          ): an integer
+          ): a non-negative integer or -1
         </h1>
         <dl class="header">
         </dl>
@@ -1188,6 +1188,30 @@ <h1>
           <p>This algorithm always returns -1 if _fromIndex_ + the length of _searchValue_ > the length of _string_.</p>
         </emu-note>
       </emu-clause>
+
+      <emu-clause id="sec-stringlastindexof" type="abstract operation">
+        <h1>
+          StringLastIndexOf (
+            _string_: a String,
+            _searchValue_: a String,
+            _fromIndex_: a non-negative integer,
+          ): a non-negative integer or -1
+        </h1>
+        <dl class="header">
+        </dl>
+        <emu-alg>
+          1. Let _len_ be the length of _string_.
+          1. Let _searchLen_ be the length of _searchValue_.
+          1. Assert: _fromIndex_ + _searchLen_ ≤ _len_.
+          1. For each integer _i_ such that 0 ≤ _i_ ≤ _fromIndex_, in descending order, do
+            1. Let _candidate_ be the substring of _string_ from _i_ to _i_ + _searchLen_.
+            1. If _candidate_ is _searchValue_, return _i_.
+          1. Return -1.
+        </emu-alg>
+        <emu-note>
+          <p>If _searchValue_ is the empty String, this algorithm returns _fromIndex_. The empty String is effectively found at every position within a string, including after the last code unit.</p>
+        </emu-note>
+      </emu-clause>
     </emu-clause>
 
     <emu-clause id="sec-ecmascript-language-types-symbol-type">
@@ -34646,11 +34670,7 @@ <h1>String.prototype.lastIndexOf ( _searchString_ [ , _position_ ] )</h1>
           1. Let _len_ be the length of _S_.
           1. Let _searchLen_ be the length of _searchStr_.
           1. Let _start_ be the result of clamping _pos_ between 0 and _len_ - _searchLen_.
-          1. If _searchStr_ is the empty String, return 𝔽(_start_).
-          1. For each integer _i_ such that 0 ≤ _i_ ≤ _start_, in descending order, do
-            1. Let _candidate_ be the substring of _S_ from _i_ to _i_ + _searchLen_.
-            1. If _candidate_ is _searchStr_, return 𝔽(_i_).
-          1. Return *-1*<sub>𝔽</sub>.
+          1. Return 𝔽(StringLastIndexOf(_S_, _searchStr_, _start_)).
         </emu-alg>
         <emu-note>
           <p>This method is intentionally generic; it does not require that its *this* value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.</p>