Schema stitching fails when type/field names collide with JS built-ins

[gnidan]

When using the field name constructor in my schema, graphql-tools fails to perform transformations inside delegateToSchema: it incorrectly thinks here that the mapping already exists, and thus throws an exception.

To inline the if statement:

if (this.mapping[actualTypeName][field]) {
  this.mapping[actualTypeName][field].push(parsedFragment);
} else {
  this.mapping[actualTypeName][field] = [parsedFragment];
}

In this case, since field is equal to "constructor", the object this.mapping[actualTypeName] has a regular JS constructor by default, so the if case executes even though the else case would be more correct.

I'm about to open a PR with a possible fix, to use JS Map objects instead of plain {} objects. Please advise if another approach would be better!

Thanks!

[yaacovCR]

Should be fixed by #1336, track #1308.

Thank you for bringing attention to this.