Implement String.prototype.padStart and .padEnd in JS

Summary: Implement `String.prototype.padStart` and `String.prototype.padEnd` in JS

Reviewed By: dulinriley

Differential Revision: D18063296

fbshipit-source-id: a65587648755bd97886e887dbb8d5446a9da42b3

include/hermes/VM/NativeFunctions.def

@@ -286,7 +286,6 @@ NATIVE_FUNCTION(stringPrototypeCodePointAt)
 NATIVE_FUNCTION(stringPrototypeConcat)
 NATIVE_FUNCTION(stringPrototypeLocaleCompare)
 NATIVE_FUNCTION(stringPrototypeNormalize)
-NATIVE_FUNCTION(stringPrototypePad)
 NATIVE_FUNCTION(stringPrototypeRepeat)
 NATIVE_FUNCTION(stringPrototypeReplace)
 NATIVE_FUNCTION(stringPrototypeSubstr)
@@ -309,6 +308,7 @@ NATIVE_FUNCTION(stringPrototypeIncludesOrStartsWith)
 NATIVE_FUNCTION(stringPrototypeIndexOf)
 NATIVE_FUNCTION(stringPrototypeLastIndexOf)
 NATIVE_FUNCTION(stringPrototypeMatch)
+NATIVE_FUNCTION(stringPrototypePad)
 NATIVE_FUNCTION(stringPrototypeSearch)
 NATIVE_FUNCTION(stringPrototypeSlice)
 NATIVE_FUNCTION(stringPrototypeSplit)

lib/InternalBytecode/String.js

@@ -550,6 +550,102 @@ String.prototype.slice = function slice(start, end) {
   return HermesInternal.executeCall(_StringPrototypeSubstring, S, from, to);
 };
 
+/**
+ * Helper function for String.prototype.padStart and String.prototype.padEnd,
+ * see their docuemntations for more details.
+ * @param str a value that's coercible to string.
+ * @param maxLength maximum length of the resulting string.
+ * @param fillString string used for padding, defaults to ' ' if it's
+ *                   undefined.
+ * @param padStart boolean flag for whether the caller function is
+ *                 padStart or padEnd.
+ * @return the resulting string after padding fillString to `this` string.
+ */
+function stringPrototypePadHelper(str, maxLength, fillString, padStart) {
+  if (new.target)
+    throw _TypeError('This function cannot be used as a constructor.');
+
+  // 1. Let O be ? RequireObjectCoercible(this value).
+  if (str === undefined || str === null) {
+    throw _TypeError('Value not coercible to object');
+  }
+  // 2. Let S be ? ToString(O).
+  var S = HermesInternal.toString(str);
+
+  // 3. Let intMaxLength be ? ToLength(maxLength).
+  var intMaxLength = HermesInternal.toLength(maxLength);
+  // 4. Let stringLength be the length of S.
+  var stringLength = S.length;
+
+  // 5. If intMaxLength is not greater than stringLength, return S.
+  if (intMaxLength <= stringLength) return S;
+  // 6. If fillString is undefined, let filler be the String value consisting solely
+  //    of the code unit 0x0020 (SPACE).
+  // 7. Else, let filler be ? ToString(fillString).
+  var filler =
+    fillString === undefined ? ' ' : HermesInternal.toString(fillString);
+  var fillerLen = filler.length;
+  // 8. If filler is the empty String, return S.
+  if (fillerLen === 0) return S;
+  // 9. Let fillLen be intMaxLength - stringLength.
+  var fillLen = intMaxLength - stringLength;
+
+  // 10. Let truncatedStringFiller be the String value consisting of repeated
+  //     concatenations of filler truncated to length fillLen.
+  var truncatedLen = fillLen % fillerLen;
+  var repeatTimes = fillLen / fillerLen;
+  var truncatedStringFiller =
+    HermesInternal.executeCall(_StringPrototypeRepeat, filler, repeatTimes) +
+    HermesInternal.executeCall(
+      _StringPrototypeSubstring,
+      filler,
+      0,
+      truncatedLen,
+    );
+
+  // 11. Return the string-concatenation of truncatedStringFiller and S.
+  return padStart ? truncatedStringFiller + S : S + truncatedStringFiller;
+}
+
+/**
+ * ES10.0 21.1.3.14
+ * Pads repeated copies of fillString in front of `this` string
+ * until the length achieves maxLength.
+ * Will coerce both values to strings if they aren't already.
+ * @this a value that's coercible to string.
+ * @param maxLength maximum length of the resulting string.
+ * @param [fillString] string used for padding, defaults to ' '
+ *                     if it's undefined.
+ * @return the resulting string after padding fillString in front
+ *         of `this` string.
+ */
+String.prototype.padStart = function padStart(
+  maxLength,
+  fillString = undefined,
+) {
+  if (new.target)
+    throw _TypeError('This function cannot be used as a constructor.');
+  return stringPrototypePadHelper(this, maxLength, fillString, true);
+};
+
+/**
+ * ES10.0 21.1.3.14
+ * Pads repeated copies of fillString to the end of `this` string
+ * until the length achieves maxLength.
+ * Will coerce both values to strings if they aren't already.
+ * @this a value that's coercible to string.
+ * @param maxLength maximum length of the resulting string.
+ * @param [fillString] string used for padding, defaults to ' '
+ *                     if it's undefined.
+ * @return the resulting string after padding fillString to the end
+ *         of `this` string.
+ */
+String.prototype.padEnd = function padEnd(maxLength, fillString = undefined) {
+  if (new.target)
+    throw _TypeError('This function cannot be used as a constructor.');
+  return stringPrototypePadHelper(this, maxLength, fillString, false);
+};
+
 Object.defineProperty(String.prototype, 'charAt', builtinFuncDescriptor);
 Object.defineProperty(String.prototype, 'match', builtinFuncDescriptor);
 Object.defineProperty(String.prototype, 'search', builtinFuncDescriptor);
@@ -560,6 +656,8 @@ Object.defineProperty(String.prototype, 'indexOf', builtinFuncDescriptor);
 Object.defineProperty(String.prototype, 'lastIndexOf', builtinFuncDescriptor);
 Object.defineProperty(String.prototype, 'split', builtinFuncDescriptor);
 Object.defineProperty(String.prototype, 'slice', builtinFuncDescriptor);
+Object.defineProperty(String.prototype, 'padStart', builtinFuncDescriptor);
+Object.defineProperty(String.prototype, 'padEnd', builtinFuncDescriptor);
 
 String.prototype.charAt.prototype = undefined;
 String.prototype.match.prototype = undefined;
@@ -571,3 +669,5 @@ String.prototype.indexOf.prototype = undefined;
 String.prototype.lastIndexOf.prototype = undefined;
 String.prototype.split.prototype = undefined;
 String.prototype.slice.prototype = undefined;
+String.prototype.padStart.prototype = undefined;
+String.prototype.padEnd.prototype = undefined;

lib/InternalBytecode/header.js

@@ -22,6 +22,7 @@
   var _MathMax = Math.max;
   var _MathMin = Math.min;
   var _ObjectDefineProperty = Object.defineProperty;
+  var _StringPrototypeRepeat = String.prototype.repeat;
   var _StringPrototypeSubstring = String.prototype.substring;
   var _SymbolIterator = Symbol.iterator;
   var _SymbolMatch = Symbol.match;