endsWith, startsWith and indexOf find an empty search string

src/lib.ts

@@ -678,7 +678,8 @@ namespace __String {
     }      
 
     export function endsWith(this: string, searchString: string, endPosition = this.length): boolean {
-        if (!searchString)
+        // a null check: "" is falsy, and an empty search string is found
+        if (searchString == null)
         {
             return false;
         }
@@ -703,7 +704,8 @@ namespace __String {
     }      
 
     export function indexOf(this: string, searchString: string, position = 0): int {    
-        if (!searchString)
+        // a null check: "" is falsy, and an empty search string is found
+        if (searchString == null)
         {
             return -1;
         }
@@ -989,7 +991,8 @@ namespace __String {
     }
 
     export function startsWith(this: string, searchString: string, position = 0): boolean {
-        if (!searchString)
+        // a null check: "" is falsy, and an empty search string is found
+        if (searchString == null)
         {
             return false;
         }

tests/string_empty_search.ts

@@ -0,0 +1,31 @@
+// an empty search string is found, as in JavaScript. The guard on it was `!searchString`, a null
+// check written as truthiness: once "" was falsy (TypeScriptCompiler #509) it returned "not found".
+function main() {
+    const s = "abc";
+
+    assert(s.endsWith(""), "endsWith('')");
+    assert(s.endsWith("", 1), "endsWith('', 1)");
+    assert(s.endsWith("bc"), "endsWith('bc')");
+    assert(!s.endsWith("x"), "endsWith('x')");
+
+    assert(s.startsWith(""), "startsWith('')");
+    assert(s.startsWith("", 3), "startsWith('', 3)");
+    assert(s.startsWith("ab"), "startsWith('ab')");
+    assert(!s.startsWith("x"), "startsWith('x')");
+
+    assert(s.indexOf("") == 0, "indexOf('')");
+    assert(s.indexOf("", 2) == 2, "indexOf('', 2)");
+    assert(s.indexOf("", 5) == 3, "indexOf('', 5)");
+    assert(s.indexOf("c") == 2, "indexOf('c')");
+    assert(s.indexOf("x") == -1, "indexOf('x')");
+
+    assert(s.lastIndexOf("") == 3, "lastIndexOf('')");
+    assert(s.includes(""), "includes('')");
+
+    const empty = "";
+    assert(empty.endsWith(""), "''.endsWith('')");
+    assert(empty.startsWith(""), "''.startsWith('')");
+    assert(empty.indexOf("") == 0, "''.indexOf('')");
+
+    console.log("ALL DONE");
+}