Typescript declaration order triggers runtime error

Question

Searching for a problem I have, I found claims that the declaration order of classes doesn't matter in Typescript , and that 'forward declarations' are not necessary.

In a project I am reviewing right now, this claim doesn't seem to hold up. I broke the issue down into a simple reproducible example, where even though the compiler doesn't complain, we fail at runtime:

$ cat bug.ts
class A extends B {
    constructor(public id:number) {
        super(id);
        console.log("A():" + id);
    }
}

class B {
    constructor(public id:number) {
        console.log("B():" + id);
    }
}

var a = new A(12);

$ tsc  bug.ts
$ node  bug.js

/home/ttsiod/work/a/bug.js:4
    __.prototype = b.prototype;
                    ^
TypeError: Cannot read property 'prototype' of undefined
    at __extends (/home/ttsiod/work/a/bug.js:4:21)
    at /home/ttsiod/work/a/bug.js:8:5
    at Object.<anonymous> (/home/ttsiod/work/a/bug.js:15:3)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:901:3

Either I am missing a keyword I don't know about, or the claim of "declaration order is not important" is not as generic as one would think.

Answer 1

I matters for inheritance. It doesn't matter as long as it is defined before it is used, which is the case in the link you mentioned.

For inheritance B must be defined before A, so that A can copy over members from the prototype of B.

Also there is a distinction between "declaration" (where order never matters) and "definition" where order almost always matters except for cases like hoisting http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html