Unhandled promise rejection with a symbol crashes node

[apexskier]

• Version: v7.2.1
• Platform: Darwin Chelan.local 16.3.0 Darwin Kernel Version 16.3.0: Thu Nov 17 20:23:58 PST 2016; root:xnu-3789.31.2~1/RELEASE_X86_64 x86_64
• Subsystem: internal/process/promises.js

Rejecting a promise with a Symbol will crash node. I'd expect it to handle this, or at least to crash in a different way. The traceback indicates node is unable to convert a Symbol to a string, though this is possible both in the REPL and when using .toString().

A simple demo session:

$ node
> const s = Symbol()
undefined
> s
Symbol()
> s.toString()
'Symbol()'
> const s2 = Symbol("label")
undefined
> s2
Symbol(label)
> s2.toString()
'Symbol(label)'
> Promise.reject(s2)
Promise { <rejected> Symbol(label) }
> internal/process/promises.js:60
                              `(rejection id: ${uid}): ${reason}`);
                                                         ^

TypeError: Cannot convert a Symbol value to a string
    at emitWarning (internal/process/promises.js:60:58)
    at emitPendingUnhandledRejections (internal/process/promises.js:86:11)
    at runMicrotasksCallback (internal/process/next_tick.js:61:9)
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickDomainCallback (internal/process/next_tick.js:122:9)
1 ↩ 

Chrome (V8) handles this as I'd expect:

[TimothyGu]

The correct code would be to use ${String(reason)}, since on a spec level the String function has special handling for symbols in addition to the ToString abstract operation which template strings use, which throws on symbols.

What I'm more worried about is that we might have to conduct a comprehensive audit for template string usage in the code base for possibility of symbols, since it's possible that they are going to blow up as well.

[apexskier]

Cool. I was thinking reason.toString(), but I'll try that. I've got a PR incoming that adds a test case and fixes the issue.

[TimothyGu]

@apexskier, that wouldn't work in cases like Object.create(null) which doesn't have a toString().