What do leading labels in output mean?

In looking at an object created by a javascript library in observable, I get this output:

g = x {obj: m, attr: "min_width", default_value: ƒ(), optional: false, change: c, spec: Object}

What is the x in front telling me?

It’s the name of the constructor for that object.

For example, if you have:

class Foo {}
f = new Foo

You’ll see:

f = ▸ Foo {}

If you’re using a JavaScript library published to npm, the constructor names are often minified, which explains why you might see a nondescriptive name such as “x”.

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Incidentally I loaded the non-minified version of the library (which was also available from the cdn) and now I can see what’s going on much more clearly.