testDirectProxySet10.js

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// Assigning to a non-existing property of a plain object defines that

// property on that object, even if a proxy is on the proto chain.

// Create an object that behaves just like obj except it throws (instead of

// returning undefined) if you try to get a property that doesn't exist.

function throwIfNoSuchProperty(obj) {

    return new Proxy(obj, {

        get(t, id) {

            if (id in t)

                return t[id];

            throw new Error("no such handler method: " + id);

});

// Use a touchy object as our proxy handler in this test.

var hits = 0, savedDesc = undefined;

var touchyHandler = throwIfNoSuchProperty({

    set: undefined

});

var target = {};

var proto = new Proxy(target, touchyHandler);

var receiver = Object.create(proto);

// This assignment `receiver.x = 2` results in a series of [[Set]] calls,

// starting with:

//

// - receiver.[[Set]]()

//     - receiver is an ordinary object.

//     - This looks for an own property "x" on receiver. There is none.

//     - So it walks the prototype chain, doing a tail-call to:

// - proto.[[Set]]()

//     - proto is a proxy.

//     - This does handler.[[Get]]("set") to look for a set trap

//         (that's why we need `set: undefined` on the handler, above)

//     - Since there's no "set" handler, it tail-calls:

// - target.[[Set]]()

//     - ordinary

//     - no own property "x"

//     - tail call to:

// - Object.prototype.[[Set]]()

//     - ordinary

//     - no own property "x"

//     - We're at the end of the line: there's nothing left on the proto chain.

//     - So at last we call:

// - receiver.[[DefineOwnProperty]]()

//     - ordinary

//     - creates the property

//

// Got all that? Let's try it.

//

receiver.x = 2;

assertEq(receiver.x, 2);

var desc = Object.getOwnPropertyDescriptor(receiver, "x");

assertEq(desc.enumerable, true);

assertEq(desc.configurable, true);

assertEq(desc.writable, true);

assertEq(desc.value, 2);