可以在node.js中配置函数名称吗？

Question

I'm trying to write some javascript to run both client and server side. I'd like to dynamically rename functions (to aid debugging etc). I have the following code:

function genericFunction() {}
Object.defineProperty(genericFunction, 'name', {writable: true});
genericFunction.name = 'specificFunction';
console.log(genericFunction.name);

In chrome (v43) this outputs "specificFunction" as expected. However, in node.js (v0.10), I get:

TypeError: Cannot redefine property: name

I've done a little debugging and this seems to be down to the different between the output of

console.log(Object.getOwnPropertyDescriptor(function () {}, 'name').configurable);

On chrome it's true, but node.js outputs false.

My (very limited) understanding of the spec is that for function names, configurable should always be true. Have I missed something in reading the spec? Does node.js use a different spec which says names shouldn't be configurable? Or have I stubbled across a genuine bug?

Answer 1

The .name property on Function objects cannot be changed in node v.0.12.2 because the name property on Function objects defaults to writable: false and configurable: false so there's no way to make it writable.

This is not in agreement with ES6. Since the current implementation of Chrome is in agreement with ES6 in this regard, we can assume that node is just behind in V8 engine version and some future version will fix this. Note: The --harmony flag with node does not change this behavior either.

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Here's the technical difference between node v.0.12.2 and Chrome 44.0.2403.130 m.

In node v.0.12.2, if you do this:

function f() {}
console.log(Object.getOwnPropertyDescriptor(f, "name"));

It outputs:

{value: 'f', writable: false, enumerable: false, configurable: false}

As you can see, you cannot write to the .name property and you cannot reconfigure it to be writable.

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In Chrome 44.0.2403.130 m, you get this:

{"value":"f","writable":false,"enumerable":false,"configurable":true }

You cannot write to it by default, but because it is configurable, you could change it to writable and then change the name property after that.

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This is presumably because Chrome is running a more recent version of the V8 engine. Per the ES6 spec description , the Chrome configuration looks like the proper default.

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If you run this code in the latest Chrome, you can see that it works:

 function f() {} var alias = f; log(Object.getOwnPropertyDescriptor(f, "name")); // this name change will not work because writable === false log(alias.name); f.name = "foo"; log(alias.name); // change writable to true Object.defineProperty(f, "name", {writable: true}); log(Object.getOwnPropertyDescriptor(f, "name")); // this name change will work f.name = "foo"; log(alias.name); // utility function to put output into main window for snippet function log(args) { var str = ""; for (var i = 0; i < arguments.length; i++) { if (typeof arguments[i] === "object") { str += JSON.stringify(arguments[i]); } else { str += arguments[i]; } } var div = document.createElement("div"); div.innerHTML = str; var target = log.id ? document.getElementById(log.id) : document.body; target.appendChild(div); } 

Answer 2

While previously not specified at all, the .name property of functions was a non-standard feature in many engines, which made the property to be non-writable and non-configurable though.

With the 6th Edition of ECMAScript this has changed, and .name is now an officially specced property of function instances. However, many engines have not yet adopted this change.

That it works in your Chrome browser suggests that one of the newer versions of V8 supports this. Consider updating your node.js, or switch to io.js.

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Can function names be configured in node.js?

As you have established, no (or at least, not currently).

My (very limited) understanding of the spec is that for function names, configurable should always be true.

Yes.

Have I missed something in reading the spec?

No.

Does node.js use a different spec which says names shouldn't be configurable?

No.

Or have I stubbled across a genuine bug?

I'd call it a "missing feature", not a bug. Node doesn't claim to support ES6 yet, so while they do plan to do in the future it's not really a bug currently.