Implementing string.startsWith

sebastienros · Dec 13, 2017 · 0281b6a · 0281b6a
1 parent 2f8bc2b
commit 0281b6a
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diff --git a/Jint.Tests/Runtime/EngineTests.cs b/Jint.Tests/Runtime/EngineTests.cs
@@ -2172,6 +2172,50 @@ public void ShouldPadEnd(string source, object expected)
 
             Assert.Equal(expected, result);
         }
+
+        /// <summary>
+        /// Tests for startsWith - tests created from MDN and https://github.com/mathiasbynens/String.prototype.startsWith/blob/master/tests/tests.js
+        /// </summary>
+        [Theory]
+        [InlineData("'To be, or not to be, that is the question.'.startsWith('To be')", true)]
+        [InlineData("'To be, or not to be, that is the question.'.startsWith('not to be')", false)]
+        [InlineData("'To be, or not to be, that is the question.'.startsWith()", false)]
+        [InlineData("'To be, or not to be, that is the question.'.startsWith('not to be', 10)", true)]
+        [InlineData("'undefined'.startsWith()", true)]
+        [InlineData("'undefined'.startsWith(undefined)", true)]
+        [InlineData("'undefined'.startsWith(null)", false)]
+        [InlineData("'null'.startsWith()", false)]
+        [InlineData("'null'.startsWith(undefined)", false)]
+        [InlineData("'null'.startsWith(null)", true)]
+        [InlineData("'abc'.startsWith()", false)]
+        [InlineData("'abc'.startsWith('')", true)]
+        [InlineData("'abc'.startsWith('\0')", false)]
+        [InlineData("'abc'.startsWith('a')", true)]
+        [InlineData("'abc'.startsWith('b')", false)]
+        [InlineData("'abc'.startsWith('ab')", true)]
+        [InlineData("'abc'.startsWith('bc')", false)]
+        [InlineData("'abc'.startsWith('abc')", true)]
+        [InlineData("'abc'.startsWith('bcd')", false)]
+        [InlineData("'abc'.startsWith('abcd')", false)]
+        [InlineData("'abc'.startsWith('bcde')", false)]
+        [InlineData("'abc'.startsWith('', 1)", true)]
+        [InlineData("'abc'.startsWith('\0', 1)", false)]
+        [InlineData("'abc'.startsWith('a', 1)", false)]
+        [InlineData("'abc'.startsWith('b', 1)", true)]
+        [InlineData("'abc'.startsWith('ab', 1)", false)]
+        [InlineData("'abc'.startsWith('bc', 1)", true)]
+        [InlineData("'abc'.startsWith('abc', 1)", false)]
+        [InlineData("'abc'.startsWith('bcd', 1)", false)]
+        [InlineData("'abc'.startsWith('abcd', 1)", false)]
+        [InlineData("'abc'.startsWith('bcde', 1)", false)]
+        public void ShouldStartWith(string source, object expected)
+        {
+            var engine = new Engine();
+            var result = engine.Execute(source).GetCompletionValue().ToObject();
+
+            Assert.Equal(expected, result);
+        }
+
         [Theory]
         [InlineData("throw {}", "undefined")]
         [InlineData("throw {message:null}","null")]

diff --git a/Jint/Native/String/StringPrototype.cs b/Jint/Native/String/StringPrototype.cs
@@ -44,6 +44,7 @@ public void Configure()
             FastAddProperty("charCodeAt", new ClrFunctionInstance(Engine, CharCodeAt, 1), true, false, true);
             FastAddProperty("concat", new ClrFunctionInstance(Engine, Concat, 1), true, false, true);
             FastAddProperty("indexOf", new ClrFunctionInstance(Engine, IndexOf, 1), true, false, true);
+            FastAddProperty("startsWith", new ClrFunctionInstance(Engine, StartsWith, 1), true, false, true);
             FastAddProperty("lastIndexOf", new ClrFunctionInstance(Engine, LastIndexOf, 1), true, false, true);
             FastAddProperty("localeCompare", new ClrFunctionInstance(Engine, LocaleCompare, 1), true, false, true);
             FastAddProperty("match", new ClrFunctionInstance(Engine, Match, 1), true, false, true);
@@ -793,5 +794,53 @@ private JsValue Pad(JsValue thisObj, JsValue[] arguments, bool padStart)
 
             return padStart ? $"{padString.Substring(0, targetLength)}{s}" : $"{s}{padString.Substring(0, targetLength)}";
         }
+
+        /// <summary>
+        /// https://www.ecma-international.org/ecma-262/6.0/#sec-string.prototype.startswith
+        /// </summary>
+        /// <param name="thisObj"></param>
+        /// <param name="arguments"></param>
+        /// <returns></returns>
+        private JsValue StartsWith(JsValue thisObj, JsValue[] arguments)
+        {
+            TypeConverter.CheckObjectCoercible(Engine, thisObj);
+
+            var s = TypeConverter.ToString(thisObj);
+
+            var searchString = arguments.At(0);
+            if (searchString == Null.Instance)
+            {
+                searchString = Null.Text;
+            }
+            else
+            {
+                if (searchString.IsRegExp())
+                {
+                    throw new JavaScriptException(Engine.TypeError);
+                }
+            }
+
+            var searchStr = TypeConverter.ToString(searchString);
+
+            var pos = TypeConverter.ToInt32(arguments.At(1));
+
+            var len = s.Length;
+            var start = System.Math.Min(System.Math.Max(pos, 0), len);
+            var searchLength = searchStr.Length;
+            if (searchLength + start > len)
+            {
+                return false;
+            }
+
+            for (var i = 0; i < searchLength; i++)
+            {
+                if (s[start + i] != searchStr[i])
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
     }
 }