String.prototype.substr() support

src/js.rs

@@ -341,19 +341,26 @@ extern {
     #[wasm_bindgen(js_name = JsString)]
     pub type JsString;
 
+    /// The String object's charAt() method returns a new string consisting of the single
+    /// UTF-16 code unit located at the specified offset into the string.
+    ///
+    /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charAt
+    #[wasm_bindgen(method, js_class = "String", js_name = charAt)]
+    pub fn char_at(this: &JsString, index: u32) -> JsString;
+
     /// The slice() method extracts a section of a string and returns it as a
     /// new string, without modifying the original string.
     ///
     /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice
     #[wasm_bindgen(method, js_class = "String")]
     pub fn slice(this: &JsString, start: u32, end: u32) -> JsString;
 
-    /// The String object's charAt() method returns a new string consisting of the single
-    /// UTF-16 code unit located at the specified offset into the string.
+    /// The substr() method returns the part of a string between
+    /// the start index and a number of characters after it.
     ///
-    /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charAt
-    #[wasm_bindgen(method, js_class = "String", js_name = charAt)]
-    pub fn char_at(this: &JsString, index: u32) -> JsString;
+    /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr
+    #[wasm_bindgen(method, js_class = "String")]
+    pub fn substr(this: &JsString, start: i32, length: i32) -> JsString;
 }
 
 impl<'a> From<&'a str> for JsString {

tests/all/js_globals/JsString.rs

@@ -59,3 +59,39 @@ fn slice() {
         "#)
         .test()
 }
+
+#[test]
+fn substr() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section)]
+
+            extern crate wasm_bindgen;
+            use wasm_bindgen::prelude::*;
+            use wasm_bindgen::js;
+
+            #[wasm_bindgen]
+            pub fn create_substr(this: &js::JsString, start: i32, length: i32) -> js::JsString {
+                this.substr(start, length)
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            export function test() {
+                let aString = "Mozilla";
+
+                assert.equal(wasm.create_substr(aString, 0, 1), "M");
+                assert.equal(wasm.create_substr(aString, 1, 0), "");
+                assert.equal(wasm.create_substr(aString, -1, 1), "a");
+                assert.equal(wasm.create_substr(aString, 1, -1), "");
+                // TODO: Uncomment and test these assertions, once we have support for optional parameters
+                // assert.equal(wasm.create_substr(aString, -3), "lla");
+                // assert.equal(wasm.create_substr(aString, 1), "ozilla");
+                assert.equal(wasm.create_substr(aString, -20, 2), "Mo");
+                assert.equal(wasm.create_substr(aString, 20, 2), "");
+            }
+        "#)
+        .test()
+}