[esnext] implement String padding proposal

test/mjsunit/harmony/harmony-string-pad-start.js

Index: test/mjsunit/harmony/harmony-string-pad-start.js
diff --git a/test/mjsunit/harmony/harmony-string-pad-start.js b/test/mjsunit/harmony/harmony-string-pad-start.js
new file mode 100644
index 0000000000000000000000000000000000000000..2b2d004251a13a82fbd9a382177f8bf65ed20f4f
--- /dev/null
+++ b/test/mjsunit/harmony/harmony-string-pad-start.js
@@ -0,0 +1,87 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-string-padding
+
+(function TestMeta() {
+  assertEquals(1, String.prototype.padStart.length);
+  assertEquals("function", typeof String.prototype.padStart);
+  assertEquals(Object.getPrototypeOf(Function),
+               Object.getPrototypeOf(String.prototype.padStart));
+  assertEquals("padStart", String.prototype.padStart.name);
+
+  var desc = Object.getOwnPropertyDescriptor(String.prototype, "padStart");
+  assertFalse(desc.enumerable);
+  assertTrue(desc.configurable);
+  assertTrue(desc.writable);
+  assertEquals(undefined, desc.get);
+  assertEquals(undefined, desc.set);
+
+  assertThrows(() => new Function(`${String.prototype.padStart}`), SyntaxError);
+})();
+
+
+(function TestRequireObjectCoercible() {
+  var padStart = String.prototype.padStart;
+  assertThrows(() => padStart.call(null, 4, "test"), TypeError);
+  assertThrows(() => padStart.call(undefined, 4, "test"), TypeError);
+  assertEquals("   123", padStart.call({
+    __proto__: null,
+    valueOf() { return 123; }
+  }, 6, " "));
+
+  var proxy = new Proxy({}, {
+    get(t, name) {
+      if (name === Symbol.toPrimitive || name === "toString") return;
+      if (name === "valueOf") return () => 6.7;
+      assertUnreachable();
+    }
+  });
+  assertEquals("   6.7", padStart.call(proxy, 6, " "));
+
+  proxy = new Proxy({}, {
+    get(t, name) {
+      if (name === Symbol.toPrimitive || name === "valueOf") return;
+      if (name === "toString") return () => 6.7;
+      assertUnreachable();
+    }
+  });
+  assertEquals("   6.7", padStart.call(proxy, 6, " "));
+})();
+
+
+(function TestToLength() {
+  assertThrows(() => "123".padStart(Symbol("16")), TypeError);
+  assertEquals("123", "123".padStart(-1));
+  assertEquals("123", "123".padStart({ toString() { return -1; } }));
+  assertEquals("123", "123".padStart(-0));
+  assertEquals("123", "123".padStart({ toString() { return -0; } }));
+  assertEquals("123", "123".padStart(+0));
+  assertEquals("123", "123".padStart({ toString() { return +0; } }));
+  assertEquals("123", "123".padStart(NaN));
+  assertEquals("123", "123".padStart({ toString() { return NaN; } }));
+})();
+
+
+(function TestFillerToString() {
+  assertEquals("         .", ".".padStart(10));
+  assertEquals("         .", ".".padStart(10, undefined));
+  assertEquals("         .", ".".padStart(10, { toString() { return ""; } }));
+  assertEquals("nullnulln.", ".".padStart(10, null));
+})();
+
+
+(function TestFillerRepetition() {
+  for (var i = 2000; i > 0; --i) {
+    var expected = "xoxo".repeat(i / 4).slice(0, i - 3) + "123";
+    var actual = "123".padStart(i, "xoxo");
+    assertEquals(expected, actual);
+    assertEquals(i > "123".length ? i : 3, actual.length);
+  }
+})();
+
+
+(function TestTruncation() {
+  assertEquals("ba", "a".padStart(2, "bc"));
+})();