RegExp.compile() actually returns this, per ES6 annex B.

Arnavion · Arnavion · commit d2d02c796bea · 2016-03-22T19:49:43.000-07:00
diff --git a/src/lib/core.d.ts b/src/lib/core.d.ts
@@ -836,7 +836,7 @@ interface RegExp {
     lastIndex: number;
 
     // Non-standard extensions
-    compile(): RegExp | undefined;
+    compile(): this;
 }
 
 interface RegExpConstructor {

Original file line number	Diff line number	Diff line change
`@@ -836,7 +836,7 @@ interface RegExp {`
`836`	`836`	`lastIndex: number;`
`837`	`837`
`838`	`838`	`// Non-standard extensions`
`839`		`- compile(): RegExp \| undefined;`
	`839`	`+ compile(): this;`
`840`	`840`	`}`
`841`	`841`
`842`	`842`	`interface RegExpConstructor {`