Custom error extensions in GraphQL - Step-by-Step Execution

Concept Flow - Custom error extensions

GraphQL Query Received

↓

Execute Resolver

↓

Error Occurs?

No→Return Data

Yes↓

Create Error Object

↓

Add Custom Extensions

↓

Return Error with Extensions

When a GraphQL query runs, if an error happens, a custom extensions object can be added to the error before sending it back.

Execution Sample

GraphQL

throw new GraphQLError('Not found', {
  extensions: { code: 'NOT_FOUND', detail: 'User ID missing' }
});

This code throws a GraphQL error with a custom extensions object containing extra info.

Execution Table

Step	Action	Error Object State	Extensions Added	Result Sent
1	Query received	No error yet	None	No response yet
2	Resolver runs	No error yet	None	No response yet
3	Error thrown	Error message: 'Not found'	None	No response yet
4	Add extensions	Error message: 'Not found'	{ code: 'NOT_FOUND', detail: 'User ID missing' }	No response yet
5	Return error	Error with extensions	{ code: 'NOT_FOUND', detail: 'User ID missing' }	Error response sent

💡 Error with custom extensions returned to client

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
error	undefined	{ message: 'Not found' }	{ message: 'Not found', extensions: { code: 'NOT_FOUND', detail: 'User ID missing' } }	Returned to client with extensions

Key Moments - 2 Insights

Why do we add extensions to the error object?

Extensions provide extra structured info about the error, like error codes or details, which helps clients handle errors better. See execution_table step 4.

Does the error message change when we add extensions?

No, the main error message stays the same. Extensions add extra info without changing the message. See execution_table steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are extensions added to the error?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Extensions Added' column in execution_table rows

According to variable_tracker, what is the state of 'error' after step 3?

A { message: 'Not found', extensions: {...} }

B undefined

C { message: 'Not found' }

D Returned to client

💡 Hint

Look at the 'After Step 3' column for 'error' in variable_tracker

If we did not add extensions, what would the client receive?

A Error with only message, no extensions

B Error with default extensions

C Error with no message

D No error at all

💡 Hint

Refer to execution_table step 3 where error is thrown without extensions

Concept Snapshot

GraphQL errors can include a custom 'extensions' object.
Use 'extensions' to add extra info like error codes.
Throw errors with 'new GraphQLError(message, { extensions })'.
Clients receive both message and extensions.
Extensions help clients handle errors better.

Full Transcript

When a GraphQL query runs, the server executes resolvers. If an error happens, the server creates an error object with a message. We can add a custom 'extensions' object to this error to provide extra details like error codes or descriptions. This helps clients understand and handle errors better. The error with extensions is then sent back in the response. The main error message stays the same; extensions just add more info. This process ensures clear and structured error reporting in GraphQL APIs.