TypeError: Cannot assign to read only property 'name' of function in lib/sinon/util/core/extend.js

[jandockx]

Describe the bug

When creating a spy for a function that has an explicit name property, lib/sinon/util/core/extend.js fails on line 88:

TypeError: Cannot assign to read only property 'name' of function 'function proxy() {
                return p.invoke(func, this, slice(arguments));
            }'
    at copyValue (node_modules/sinon/lib/sinon/util/core/extend.js:88:20)
    at extendCommon (node_modules/sinon/lib/sinon/util/core/extend.js:62:17)
    at extend (node_modules/sinon/lib/sinon/util/core/extend.js:87:12)
    at createProxy (node_modules/sinon/lib/sinon/proxy.js:196:5)
    at createSpy (node_modules/sinon/lib/sinon/spy.js:122:17)
    at Function.spy (node_modules/sinon/lib/sinon/spy.js:157:45)
    at Sandbox.spy (node_modules/sinon/lib/sinon/sandbox.js:322:35)
    at Context.<anonymous> (test/FlowLogger.test.js:168:44)
    at processImmediate (internal/timers.js:439:21)

The reason is that proxy.name is not writeable.

To Reproduce

Try to spy on a function that has a name as an own property.

  it('cannot spy', function () {
    const brandNewName = 'my brand new name'
    function importantFunction () {}

    importantFunction.name.should.equal('importantFunction')

    Object.defineProperty(importantFunction, 'name', {
      configurable: false,
      enumerable: true,
      value: brandNewName,
      writable: false
    })

    console.log(importantFunction)
    importantFunction.name.should.equal(brandNewName)

    function callSomethingImportant () {
      importantFunction()
    }

    const sandbox = sinon.createSandbox()
    const stub = sandbox.spy(importantFunction) // fails

    callSomethingImportant()
    stub.should.be.calledOnce()
  })

Expected behavior

Make the proxy have the same name, or skip copying the name to the proxy.

Solution

I believe you can do this, by not copying with dest[prop] = source[prop], but by defining the property on dest as derived:

    return extendCommon(target, sources, function copyValue(dest, source, prop) {
        const sourcePropDescriptor = Object.getOwnPropertyDescriptor(source, prop)
        Object.defineProperty(dest, prop, {
            configurable: sourcePropDescriptor.configurable,
            enumerable: true, // otherwise we would not try to do this: we are iterating over enumerable own properties
            writable: sourcePropDescriptor.writable,
            value: source[prop]
        })
    });

If that's not what you want to do, you can skip copying properties that are not writeable on the target function. name is the only one I can come up with for a function, so, since this is a border case, that might be good enough (but less ideal):

    return extendCommon(target, sources, function copyValue(dest, source, prop) {
        if (prop !== 'name') {
            dest[prop] = source[prop];
        }
    });

Or, combine both approaches, for performance reasons:

    return extendCommon(target, sources, function copyValue(dest, source, prop) {
        if (prop !== 'name') {
            const sourcePropDescriptor = Object.getOwnPropertyDescriptor(source, prop)
            Object.defineProperty(dest, prop, {
                configurable: sourcePropDescriptor.configurable,
                enumerable: true, // otherwise we would not try to do this: we are iterating over enumerable own properties
                writable: sourcePropDescriptor.writable,
                value: source[prop]
            })
        }
        else {
            dest[prop] = source[prop];
        }
    });

Context (please complete the following information):

• Library version: 8.1.1
• Environment: Node 12.14.0
• Example URL: N/A
• Other libraries you are using: @toryt/contracts-iv

Additional context
N/A

[stale]

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[jandockx]

Activity