String default value in arrow function breaks highlighting

[j-f1]

See the reproduction here:

Syntax highlighting broken by string default parameter value in arrow function

Variables are not properly highlighted as defined.

[tom]

This one’s an upstream bug that is present in CodeMirror. Unfortunately, because arrow functions are very context-dependent (because (foo, bar='default') is a valid javascript expression and we only know that it’s a parameter list after encountering =>, and CodeMirror’s syntax highlighting system is very context-independent, I’m not sure that there’s a clean fix before CodeMirror 6.

[j-f1]

Why does it work with a number then?

[bgchen]

I haven’t tested this (so take this with a grain of salt), but I suspect this line in CodeMirror’s JS mode is causing the arrow-function detection to fail. It looks like it was added in response to this issue.

EDIT: As evidence for this, note that using backticks around your default string argument causes syntax highlighting to work again. Try pasting the following into a notebook:

test4 = (foo, bar = `string`) => [foo, bar]

[j-f1]

Reported to CodeMirror:

Issue: [JS] String in default value of parameter throws off arrow function check

[j-f1]

This was fixed in codemirror/CodeMirror@4c30e11. It looks like that commit hasn’t yet been published to npm yet, but you should be able to install from the master branch on GitHub to get the fix.

[jashkenas]

Tom backported the CodeMirror fix, so this ought to be highlighting correctly now.

Thanks, Tom!

[j-f1]

Awesome. Thank you!