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TypeScript 2.1 introduced a number of breaking changes, among them that “Extending built-ins like Error, Array, and Map may no longer work”. For a subclass like the following: class FooError extends Error { constructor(m: string) { super(m); } sayHello() { return "hello " + this.message; } } methods may be undefined on objects returned by constructing these subclasses, so calling sayHello will result in an error. instanceof will be broken between instances of t...