facebook/relay · error

Expected the JS type for

Error message

Expected the JS type for '{}' to be defined, please update 'customScalarTypes' in your compiler config.

What it means

For custom scalars, typegen must map the GraphQL scalar to a JS type via customScalarTypes. When the mapping is missing and the project's typegen config sets require_custom_scalar_types, the compiler panics, telling you to declare the JS type in your compiler config.

Solutions

1. Add the scalar to customScalarTypes in your relay compiler config with its JS type path/export.
2. Confirm the config key exactly matches the schema scalar name (case-sensitive).
3. Set require_custom_scalar_types: false only as a temporary measure to fall back to Any.
4. Regenerate with the updated config and verify all custom scalars are covered.

Example fix

// relay.config.js before
customScalarTypes: {},
// after
customScalarTypes: {
  DateTime: { path: './src/types/DateTime.ts', export: 'DateTime' },
},

// ensure every schema custom scalar has a JS type mapping
for (const scalar of getCustomScalars(schema)) {
  if (!config.customScalarTypes[scalar.name]) throw new Error(`Missing customScalarTypes entry for ${scalar.name}`);
}

const isMappedScalar = (name, cfg) => Object.prototype.hasOwnProperty.call(cfg.customScalarTypes ?? {}, name);

When it happens

Trigger: transform_graphql_scalar_type resolves a scalar whose name is absent from the customScalarTypes mapping, with project_config.typegen_config.require_custom_scalar_types == true; triggered from transform_non_nullable_input_type / expect_scalar_type / return_ast_in_object_case.

Common situations: Adding a new custom scalar to the schema without updating customScalarTypes in relay.config; enabling require_custom_scalar_types in a project with legacy unmapped scalars; typos between the schema scalar name and the config key.

Related errors

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• Expected to have a typegen operation for

AI-assisted analysis of facebook/relay@668b1b85e0 (2026-09-02). Data as JSON: /api/errors/31376833355369db. Report an issue: GitHub.

Appendix: source

Thrown at compiler/crates/relay-typegen/src/visit.rs:2353

            CustomType::Path(CustomTypeImport { name, path }) => {
                custom_scalars.insert((*name, path.clone()));

                AST::RawType(*name)
            }
        }
    } else if scalar_name.item == *TYPE_ID || scalar_name.item == *TYPE_STRING {
        AST::String
    } else if scalar_name.item == *TYPE_FLOAT || scalar_name.item == *TYPE_INT {
        AST::Number
    } else if scalar_name.item == *TYPE_BOOLEAN {
        AST::Boolean
    } else {
        if typegen_context
            .project_config
            .typegen_config
            .require_custom_scalar_types
        {
            panic!(
                "Expected the JS type for '{}' to be defined, please update 'customScalarTypes' in your compiler config.",
                scalar_name.item
            );
        }
        AST::Any
    }
}

fn transform_graphql_enum_type(
    schema: &SDLSchema,
    enum_id: EnumID,
    encountered_enums: &mut EncounteredEnums,
) -> AST {
    encountered_enums.0.insert(enum_id);
    AST::Identifier(schema.enum_(enum_id).name.item.0)
}

#[allow(clippy::too_many_arguments)]

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