Bug 1658308 - Implement String.prototyp.item in Nightly. r=anba

Differential Revision: https://phabricator.services.mozilla.com/D92640

js/src/builtin/String.cpp

@@ -3639,6 +3639,10 @@ static const JSFunctionSpec string_methods[] = {
     JS_SELF_HOSTED_FN("fontcolor", "String_fontcolor", 1, 0),
     JS_SELF_HOSTED_FN("fontsize", "String_fontsize", 1, 0),
 
+#ifdef NIGHTLY_BUILD
+    JS_SELF_HOSTED_FN("item", "String_item", 1, 0),
+#endif
+
     JS_SELF_HOSTED_SYM_FN(iterator, "String_iterator", 0, 0), JS_FS_END};
 
 // ES6 rev 27 (2014 Aug 24) 21.1.1

js/src/builtin/String.js

@@ -888,6 +888,39 @@ function String_toLocaleUpperCase() {
 }
 #endif  // JS_HAS_INTL_API
 
+// https://github.com/tc39/proposal-item-method
+// String.prototype.item ( index )
+function String_item(index) {
+    // Step 1.
+    if (this === undefined || this === null)
+        ThrowIncompatibleMethod("item", this);
+
+    // Step 2.
+    var str = ToString(this);
+
+    // Step 3.
+    var len = str.length;
+
+    // Step 4.
+    var relativeIndex = ToInteger(index);
+
+    // Steps 5-6.
+    var k;
+    if (relativeIndex >= 0) {
+        k = relativeIndex;
+    } else {
+        k = len + relativeIndex;
+    }
+
+    // Step 7.
+    if (k < 0 || k >= len) {
+        return undefined;
+    }
+
+    // Step 8.
+    return str[k];
+}
+
 // ES2018 draft rev 8fadde42cf6a9879b4ab0cb6142b31c4ee501667
 // 21.1.2.4 String.raw ( template, ...substitutions )
 function String_static_raw(callSite/*, ...substitutions*/) {

js/src/tests/non262/String/item.js

@@ -0,0 +1,30 @@
+// |reftest| skip-if(!String.prototype.item)
+
+function basic() {
+  assertEq("a".item(0), "a");
+  assertEq("a".item(-1), "a");
+
+  assertEq("".item(0), undefined);
+  assertEq("".item(-1), undefined);
+  assertEq("".item(1), undefined);
+
+  assertEq("ab".item(0), "a");
+  assertEq("ab".item(1), "b");
+  assertEq("ab".item(-2), "a");
+  assertEq("ab".item(-1), "b");
+
+  assertEq("ab".item(2), undefined);
+  assertEq("ab".item(-3), undefined);
+  assertEq("ab".item(-4), undefined);
+  assertEq("ab".item(Infinity), undefined);
+  assertEq("ab".item(-Infinity), undefined);
+  assertEq("ab".item(NaN), "a"); // ToInteger(NaN) = 0
+
+  assertEq("\u{1F921}".item(0), "\u{D83E}");
+  assertEq("\u{1F921}".item(1), "\u{DD21}");
+}
+
+basic();
+
+if (typeof reportCompare === "function")
+    reportCompare(true, true);