String: substr

src/lib.d.ts

@@ -340,6 +340,8 @@ declare namespace __String {
 
     function split(this: string, separator?: string | RegExp, limit?: int): string[];
 
+    function substr(this: string, start: int, length?: int): string;
+
     function substring(this: string, indexStart: int, indexEnd?: int): string;
 
     function startsWith(this: string, searchString: string, position?: int): boolean;
@@ -416,6 +418,8 @@ declare class String {
 
     split(this: string, separator?: string | RegExp, limit?: int): string[];
 
+    substr(this: string, start: int, length?: int): string;
+
     substring(this: string, indexStart: int, indexEnd?: int): string;
 
     startsWith(this: string, searchString: string, position?: int);

src/lib.ts

@@ -1181,6 +1181,28 @@ namespace __String {
         return strncmp(<string>Ref(this[position]), searchString, lensuffix) == 0;
     }    
 
+    // `length` characters from `start`; a negative start counts from the end
+    export function substr(this: string, start: int, length: int = this.length): string {
+        if (start < 0) {
+            start = start + this.length;
+            if (start < 0) {
+                start = 0;
+            }
+        } else if (start >= this.length) {
+            return "";
+        }
+
+        if (length <= 0) {
+            return "";
+        }
+
+        if (length > this.length - start) {
+            length = this.length - start;
+        }
+
+        return this.substring(start, start + length);
+    }
+
     export function substring(this: string, indexStart: int, indexEnd: int = this.length): string {
         if (indexStart < 0) {
             indexStart = 0;
@@ -1392,6 +1414,10 @@ export class String {
         return this.value.split(separator, limit);
     }
 
+    public substr(start: int, length?: int): string {
+        return this.value.substr(start, length);
+    }
+
     public substring(indexStart: int, indexEnd?: int): string {
         return this.value.substring(indexStart, indexEnd);
     }

tests/string_substr.ts

@@ -0,0 +1,19 @@
+function main() {
+    const s = "Mozilla";
+
+    assert(s.substr(0, 1) == "M");
+    assert(s.substr(1, 0) == "");
+    assert(s.substr(-1, 1) == "a");
+    assert(s.substr(1, -1) == "");
+    assert(s.substr(-3) == "lla");
+    assert(s.substr(1) == "ozilla");
+    assert(s.substr(-20, 2) == "Mo");
+    assert(s.substr(20, 2) == "");
+    assert(s.substr(2, 100) == "zilla");
+
+    // through the String wrapper
+    const w = new String("Mozilla");
+    assert(w.substr(2, 3) == "zil");
+
+    console.log("ALL DONE");
+}