Nullability Guard

Nullability Guard

This plugin helps us guard against non-null values crashing our queries in production. It does this by defining that every scalar value have a "fallback" value defined, so if we see a nullish value on an otherwise non-null field, we will fallback to this instead of crashing the query.

The nullabilityGuardPlugin by default only guards when process.env.NODE_ENV === 'production'. This is intended so you see/catch these errors in development and do not use it except as a last resort. If you want to change this, set the shouldGuard config option.


1import { nullabilityGuardPlugin } from 'nexus'
3const guardPlugin = nullabilityGuardPlugin({
4 onGuarded({ ctx, info }) {
5 // This could report to a service like Sentry, or log internally - up to you!
6 console.error(
7 `Error: Saw a null value for non-null field ${}.${info.fieldName} ${
8 root ? `(${ || root._id})` : ''
9 }`
10 )
11 },
12 // A map of `typeNames` to the values we want to replace with if a "null" value
13 // is seen in a position it shouldn't be. These can also be provided as a config property
14 // for the `objectType` / `enumType` definition, as seen below.
15 fallbackValues: {
16 Int: () => 0,
17 String: () => '',
18 ID: ({ info }) => `${}:N/A`,
19 Boolean: () => false,
20 Float: () => 0,
21 },

Null Guard Algorithm

  • If a field is nullable:
    • If the field is non-list, do not guard
    • If the field is a list, and none of the list members are nullable, do not guard
  • If the field is non-nullable and the value is null:
    • If the field is a list:
      • If the value is nullish, return an empty list []
      • If the list is non-empty, iterate and complete with a valid non-null fallback
    • If the value is a Union/Interface
      • Return with an object with the __typename of the first type which implements this contract
    • If the field is an object:
      • If the value is nullish
        • If there is a fallback defined on the object for that type, return with that
        • Else return with an empty object
      • Return the value and push forward to the next resolvers
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