require("v8").setFlagsFromString('--expose_gc'); not setting --expose_gc flag

[natewatson999]

• Version: 8.8.1
• Platform: Win64
• Subsystem: v8 module

There is an issue in this version of Node, where --expose_gc works as a command line option, but not when used by v8.setFlagsFromString:

When one uses this input:

$>node --expose_gc
js>typeof global.gc

The output is:

'function'

In contrast, when one uses this:

$>node
js>require("v8").setFlagsFromString('--expose_gc');
js>typeof global.gc

The output is:

undefined

The output should be 'function', since this should enable the global.gc function.

[addaleax]

This would require setFlagsFromString to modify the existing global object, which I assume V8 would not really be willing to do.

But, I think this should be a pretty good workaround: You can create new global objects using the vm module, so

require("v8").setFlagsFromString('--expose_gc');
global.gc = require("vm").runInNewContext('gc');

might do what you want?

[bnoordhuis]

I'll close this out. @addaleax's workaround should do the trick and if not, this sentence from the documentation applies: "it may simply do nothing."

[natewatson999]

@addaleax , I have a question.
I understand that creates a new VM context. If I use that code, and call global.gc; will that trigger garbage collection in all of the contexts, or just the one created there with the vm module?

[addaleax]

@natewatson999 All of the contexts, the functions will behave identically.

[liushiqi9]

I think the main reason is because the --expose_gc flag need to be set before the Isolate is constructed. If you set the flag after the Isolate is constructed, it's useless.