Implement array.flat and array.flatMap

js/src/builtin/Array.js

@@ -1073,6 +1073,114 @@ function ArrayConcat(arg1) {
     return A;
 }
 
+// https://tc39.github.io/proposal-flatMap/
+// January 4, 2019
+function ArrayFlatMap(mapperFunction/*, thisArg*/) {
+    // Step 1.
+    var O = ToObject(this);
+
+    // Step 2.
+    var sourceLen = ToLength(O.length);
+
+    // Step 3.
+    if (!IsCallable(mapperFunction))
+        ThrowTypeError(JSMSG_NOT_FUNCTION, DecompileArg(0, mapperFunction));
+
+    // Step 4.
+    var T = arguments.length > 1 ? arguments[1] : undefined;
+
+    // Step 5.
+    var A = ArraySpeciesCreate(O, 0);
+
+    // Step 6.
+    FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction, T);
+
+    // Step 7.
+    return A;
+}
+
+// https://tc39.github.io/proposal-flatMap/
+// January 4, 2019
+function ArrayFlat(/* depth */) {
+     // Step 1.
+    var O = ToObject(this);
+
+    // Step 2.
+    var sourceLen = ToLength(O.length);
+
+    // Step 3.
+    var depthNum = 1;
+
+    // Step 4.
+    if (arguments.length > 0 && arguments[0] !== undefined)
+        depthNum = ToInteger(arguments[0]);
+
+    // Step 5.
+    var A = ArraySpeciesCreate(O, 0);
+
+    // Step 6.
+    FlattenIntoArray(A, O, sourceLen, 0, depthNum);
+
+    // Step 7.
+    return A;
+}
+
+// https://tc39.github.io/proposal-flatMap/
+// January 4, 2019
+function FlattenIntoArray(target, source, sourceLen, start, depth, mapperFunction, thisArg) {
+    // Step 1.
+    var targetIndex = start;
+
+    // Steps 2-3.
+    for (var sourceIndex = 0; sourceIndex < sourceLen; sourceIndex++) {
+        // Steps 3.a-c.
+        if (sourceIndex in source) {
+            // Step 3.c.i.
+            var element = source[sourceIndex];
+
+            if (mapperFunction) {
+                // Step 3.c.ii.1.
+                assert(arguments.length === 7, "thisArg is present");
+
+                // Step 3.c.ii.2.
+                element = callContentFunction(mapperFunction, thisArg, element, sourceIndex, source);
+            }
+
+            // Step 3.c.iii.
+            var shouldFlatten = false;
+
+            // Step 3.c.iv.
+            if (depth > 0) {
+                // Step 3.c.iv.1.
+                shouldFlatten = IsArray(element);
+            }
+
+            // Step 3.c.v.
+            if (shouldFlatten) {
+                // Step 3.c.v.1.
+                var elementLen = ToLength(element.length);
+
+                // Step 3.c.v.2.
+                // Recursive call to walk the depth.
+                targetIndex = FlattenIntoArray(target, element, elementLen, targetIndex, depth - 1);
+            } else {
+                // Step 3.c.vi.1.
+                if (targetIndex >= MAX_NUMERIC_INDEX)
+                    ThrowTypeError(JSMSG_TOO_LONG_ARRAY);
+
+                // Step 3.c.vi.2.
+                _DefineDataProperty(target, targetIndex, element);
+
+                // Step 3.c.vi.3.
+                targetIndex++;
+            }
+        }
+    }
+
+    // Step 4.
+    return targetIndex;
+}
+
 function ArrayStaticConcat(arr, arg1) {
     if (arguments.length < 1)
         ThrowTypeError(JSMSG_MISSING_FUN_ARG, 0, 'Array.concat');

js/src/jsarray.cpp

@@ -3169,6 +3169,11 @@ static const JSFunctionSpec array_methods[] = {
 
     /* ES7 additions */
     JS_SELF_HOSTED_FN("includes",    "ArrayIncludes",    2,0),
+    
+    /* ES2019 additions */
+    JS_SELF_HOSTED_FN("flat",        "ArrayFlat",        0,0),
+    JS_SELF_HOSTED_FN("flatMap",     "ArrayFlatMap",     1,0),
+    
     JS_FS_END
 };
 
@@ -3333,6 +3338,8 @@ array_proto_finish(JSContext* cx, JS::HandleObject ctor, JS::HandleObject proto)
         !DefineProperty(cx, unscopables, cx->names().fill, value) ||
         !DefineProperty(cx, unscopables, cx->names().find, value) ||
         !DefineProperty(cx, unscopables, cx->names().findIndex, value) ||
+        !DefineProperty(cx, unscopables, cx->names().flat, value) ||
+        !DefineProperty(cx, unscopables, cx->names().flatMap, value) ||
         !DefineProperty(cx, unscopables, cx->names().includes, value) ||
         !DefineProperty(cx, unscopables, cx->names().keys, value) ||
         !DefineProperty(cx, unscopables, cx->names().values, value))

js/src/tests/ecma_6/Array/unscopables.js

@@ -26,6 +26,8 @@ assertDeepEq(keys, [
     "fill",
     "find",
     "findIndex",
+    "flat",
+    "flatMap",
     "includes",
     "keys",
     "values"

js/src/vm/CommonPropertyNames.h

@@ -115,6 +115,8 @@
     macro(firstDayOfWeek, firstDayOfWeek, "firstDayOfWeek") \
     macro(fix, fix, "fix") \
     macro(flags, flags, "flags") \
+    macro(flat, flat, "flat") \
+    macro(flatMap, flatMap, "flatMap") \
     macro(float32, float32, "float32") \
     macro(Float32x4, Float32x4, "Float32x4") \
     macro(float64, float64, "float64") \

js/xpconnect/tests/chrome/test_xrayToJS.xul

@@ -198,7 +198,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       "pop", "shift", "unshift", "splice", "concat", "slice", "lastIndexOf", "indexOf",
       "includes", "forEach", "map", "reduce", "reduceRight", "filter", "some", "every", "find",
       "findIndex", "copyWithin", "fill", Symbol.iterator, Symbol.unscopables, "entries", "keys",
-      "values", "constructor"];
+      "values", "constructor", "flat", "flatMap"];
   if (isNightlyBuild) {
     // ...nothing now
   }