Contexts created with vm.createContext() do not define the URL() constructor

• Version: v12.6.0 (also seen in 10.13.0)
• Platform: Darwin Davids-MacBook-Pro.local 18.5.0 Darwin Kernel Version 18.5.0: Mon Mar 11 20:40:32 PDT 2019; root:xnu-4903.251.3~3/RELEASE_X86_64 x86_64
• Subsystem:

I've created a simple testing framework that runs tests using vm.Script.runInContext(). Now I'm writing tests for code that uses the whatwg URL API. If I use vm.createContext(), the created context does not define the URL() constructor. But if I pass in the URL constructor with vm.createContext({URL}), then I have a situation where arrays returned by URLSearchParams methods are defined using the Array.prototype object from outside the context, and my tests are trying to compare those to arrays defined inside the context with a different Array.prototype object. So because I have two arrays with different prototypes, assert.deepStrictEqual() thinks they are not the same.

I'd argue that the underlying bug here is that URL should be automatically defined in newly created contexts without having to be passed in. Or maybe this is a bug in assert.deepStrictEqual() and it is stricter than it ought to be in this cross-context situation?

In any case, here is an example that reproduces the issue for me:

const vm = require('vm');

// URL is not defined inside the context, and I can't require it, so
// I need to pass it to the context from outside. But it returns arrays
// using the Array class from outside the context.
let context = vm.createContext({require, URL, externalArray:Array});

let script = new vm.Script(`
    const assert = require('assert');
    let url = new URL('http://example.com');
    url.searchParams.append('x', '1');
    url.searchParams.append('x', '2');
    let actual = url.searchParams.getAll('x'); // Uses array class from outside
    let expected = ['1', '2'];                 // Uses array class from inside
    assert(Array.isArray(actual));                                // passes
    assert.deepStrictEqual(Array.from(actual), expected);         // passes
    assert.deepStrictEqual(actual, externalArray.from(expected)); // passes
    assert.deepStrictEqual([...actual], expected);                // passes
    assert.deepStrictEqual(actual, expected);               // fails
    assert.equal(Object.getPrototypeOf(actual),             // also fails
                 Object.getPrototypeOf(expected)); 
`);

script.runInContext(context);

issue にある vm.createContext() と vm.Script.runInContext() の再現から始めます。URL の利用可能性に関するコンテキスト初期化の動作と、コンテキストをまたいだ配列プロトタイプの結果を確認し、その後、どの動作を対象にすべきかを判断します。選択した URL または deepStrictEqual の動作に対する回帰テストと合意済みの修正があれば完了です。