vm instanceof operator don't work as expected

[davidarias]

The following code returns false

vm.runInNewContext("a instanceof Array", {a:[]})

[ghost]

Not a bug. New context and old context have each their own, distinct, Array object.

[isaacs]

Yes. Not sharing the global builtins is the point of running in a new context ;)

If you want to share specific globals, you can do it this way:

vm.runInContext("a instanceof Array", vm.createContext({a:[], Array:Array}))

EDIT I guess this way is easier, and closer to the original code:

vm.runInNewContext("a instanceof Array", {a:[], Array:Array})

[ghost]

But if it is about (cross-context) Array testing, Array.isArray(a) is the way to go (unless I am mistaken, but I was told it works over context boundary as well).

[isaacs]

Yes, Array.isArray() is the best way to test if something is an array.

[davidarias]

Thank you very much people, that clears my trouble :)

[bpaf]

Consider the following:

https://github.com/visionmedia/express/blob/master/examples/route-loading/boot.js

https://github.com/visionmedia/express/blob/master/lib/router/route.js#L71

app.get(/^\/foo\/…$/, …)

One work-around is passing RegExp:RegExp in the context and doing:

app.get(new RegExp('^\\/foo\\/…$'), …)

but is there a way to make it work with literals?

[bnoordhuis]

@bpaf: Like this?

> require('vm').runInNewContext('re instanceof RegExp', {re:/x/})
false
> require('vm').runInNewContext('new RegExp(re) instanceof RegExp', {re:/x/})
true

[marcopaz]

actually there seems to be a bug here, node does not use the RegExp object passed in the context to create regexp objects from regexp literals.

[bnoordhuis]

@brokendroid: To quote the Zen of Python, explicit is better than implicit.

[isaacs]

Works as designed.

> vm.runInNewContext("re instanceof RegExp", {re: /a/, RegExp: RegExp})
true

Same as the Array case.