Set default value for contextValue

[mattkrick]

Following up from #326 (comment) often times it does make sense that the contextValue is an object. This is because in addition to an authToken, you might have another token (resetPasswordToken, verifyEmailToken, etc) or even something like a socket for subscriptions. However, if you don't need any of those things, it's still nice to call graphql(Schema, query) instead of graphql(Schema, query, null, {}). To keep it that way, execute should default the value to an object here: https://github.com/graphql/graphql-js/blob/master/src/execution/execute.js#L115

Thoughts?

[leebyron]

I'm not sure I follow. If you don't need any of the things you would put in context, why do you want an object? Also, if you want the empty object, is there something wrong with providing that directly?

[mattkrick]

Without an implied guarantee of an object we either need to assure it's there:
graphql(Schema, query, null, {})

Or, we need to make sure to code a little more defensively in the resolve functions.
For example, in the previous version, a standard resolve might look like:
resolve(source,args, {rootValue: {authToken}})

This was great because the rootValue is guaranteed to be an object.

If we try something similar without an object present:
resolve(source, args, {authToken}) // Broken

[leebyron]

I think if you have any usage of context or rootValue in your schema, then it's reasonable to assume you're providing something for them when executing, and so it's not necessary to code defensively in resolution functions.

Also in the previous version where rootValue was used in place of context, this situation still existed since rootValue also does not have a default value and is undefined if not provided, so resolve(source,args, {rootValue: {authToken}}) could still be considered broken if a rootValue was not provided.

[Kosta-Github]

BTW, the documentation of the contextValue (https://github.com/graphql/graphql-js/blob/master/src/graphql.js#L47) is missing in the comment right above the function graphql() (https://github.com/graphql/graphql-js/blob/master/src/graphql.js#L28-L41)...