Cannot read property 'prototype' of undefined

[nin-jin]

script.ts

class A extends B {}
class B { static foo() {} }

Produces broken js that throws subject exception

var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};
var A = (function (_super) {
    __extends(A, _super);
    function A() {
        _super.apply(this, arguments);
    }
    return A;
}(B));
var B = (function () {
    function B() {
    }
    B.foo = function () { };
    return B;
}());

We need to produce js like this

var __extends = (this && this.__extends) || function (d, b) {
    if( Object.setPrototypeOf ) Object.setPrototypeOf( d , b )
    else for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};
function A() {
    B.apply(this);
}
__extends(A, B);
function B() {
}
B.foo = function () { };

1. This is short and clear code.
2. This code are close to code writed by human.
3. This works right in modern browsers.

[kitsonk]

Classes are not hoisted (just like let and const), as per the ES2015 spec and so your code does not work in modern browsers. In Chrome it produces Uncaught ReferenceError: B is not defined.

Currently TypeScript does not warn you of this, but this is covered by #5207 and planned for TypeScript 2.1.