You can use + to produce better formatting:

`Cannot return null for non-nullable field ` +
`${info.parentType.name}.${info.fieldName}. ` +
`Value ${inspect(result)} cannot be converted to ${inspect(returnType)}`

I also think converted is not a correct term here, the spec calls it coercion:
https://facebook.github.io/graphql/draft/#example-cb7e7
Also, see 4a here:
https://facebook.github.io/graphql/draft/#CompleteValue()

Cannot return null for non-nullable field DataType.test. Value null cannot be converted to [Int!]! is strange error.
You can do:

if (result === null) {
  throw new Error(`Cannot return null for non-nullable field ${info.parentType.name}.${info.fieldName}.`);
} else {
  throw new Error(`<Another error specifically about coercion>`);
}

Thanks for the advice, I'll do that.

How does this look?

if (result === null) {
  throw new Error(
    `Cannot return null for non-nullable field ` +
      `${info.parentType.name}.${info.fieldName}.`,
  );
} else {
  throw new Error(
    `Cannot coerce '${inspect(result)}' into type ` +
      `${inspect(returnType)} for non-nullable field ` +
      `${info.parentType.name}.${info.fieldName}`,
  );
}

@domi7777 I still not sure about couple edge cases that could mismatch with this error message. I need to refresh my knowledge of this code to be sure so I will try to do that on weekend.

@IvanGoncharov - mind sharing some edge cases you can imagine?

@Neitsch @domi7777 Hi sorry for the radio silence.
I have some project that I need to finish.
I will try to review and merge all stale PRs in the next couple of weeks.

mind sharing some edge cases you can imagine?

To be honest I already forgot. I will try to find some time next week to properly review it. Also feel free to ping me.