Spying on objects with Symbol method names breaks assertion error messages

[fongandrew]

• Sinon version : 4.1.3
• Environment : Ubuntu

Code for formatting assertion error messages does not properly account for non-string method names (i.e. symbols). This may be a regression of #1002.

Reproduction Steps

Running this bit of code:

const sinon = require('sinon');
const sym = Symbol('my symbol');
const obj = { [sym]: (a, b) => a + b };
const spy = sinon.spy(obj, sym)
sinon.assert.calledWith(spy, 1, 2);

Results in this error:

TypeError: Cannot convert a Symbol value to a string
    at String.replace (<anonymous>)
    at Function.printf (/path/to/node_modules/sinon/lib/sinon/spy.js:375:31)
    at Object.assert.(anonymous function) [as calledWith] (/path/to/node_modules/sinon/lib/sinon/assert.js:80:68)

Instead of the expected AssertError: expected Symbol(my symbol) to be called with arguments.

Possible fix would be to explicitly type-cast symbol to string at

sinon/lib/sinon/spy.js

Line 379 in 5e131b3

return formatter.call(null, spyInstance, args);

(i.e. return String(formatter.call(null, spyInstance, args)))

[mroderick]

Thank you for this very thorough bug report!

Would you be up for creating a pull request to fix it?

[fongandrew]

Created!

[mroderick]

Thank you for your contribution 👍