Implement array.flat and array.flatMap

Self-hosted implementation that adds both functions and adds them to
@@unscopables as specced in ES2019.
Resolves #1095

4 files changed, 119 insertions, 0 deletions

diff --git a/js/src/builtin/Array.js b/js/src/builtin/Array.js
index 30e6fb35f..05fc41bc1 100644
--- a/js/src/builtin/Array.js
+++ b/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');
diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 7a67c0095..3b4c957dc 100644
--- a/js/src/jsarray.cpp
+++ b/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))
diff --git a/js/src/tests/ecma_6/Array/unscopables.js b/js/src/tests/ecma_6/Array/unscopables.js
index 6685309a0..56b3aa520 100644
--- a/js/src/tests/ecma_6/Array/unscopables.js
+++ b/js/src/tests/ecma_6/Array/unscopables.js
@@ -26,6 +26,8 @@ assertDeepEq(keys, [
     "fill",
     "find",
     "findIndex",
+    "flat",
+    "flatMap",
     "includes",
     "keys",
     "values"
diff --git a/js/src/vm/CommonPropertyNames.h b/js/src/vm/CommonPropertyNames.h
index e971dc844..8a36df083 100644
--- a/js/src/vm/CommonPropertyNames.h
+++ b/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") \