Ajv: Using JSONSchemaType<T> requires strictNullChecks option

[GabDug]

What version of Ajv are you using? Does the issue happen if you use the latest version?

I'm using the latest v7 beta 7, with TS 4.1.

Your typescript code

Running the example code from the README, I get types errors.

First error is with the missing nullable: true property. When adding it, the error disappears, but the validation now accepts null.
nullable: false is not a valid property neither.

Second error is regarding to the required property, which seems not to accept string[] but only never[].

Same thing happens with my real project.

Link to a REPL reproduction

import Ajv, {JSONSchemaType, DefinedError} from "ajv"

const ajv = new Ajv()

type MyData = {foo: number}

// optional schema type annotation for schema to match MyData type
const schema: JSONSchemaType<MyData> = {
  type: "object",
  properties: {
    foo: {type: "number", minimum: 0},
  },
  required: ["foo"],
  additionalProperties: false,
}

// validate is a type guard for MyData - type is inferred from schema type
const validate = ajv.compile(schema)

// or, if you did not use type annotation for the schema,
// type parameter can be used to make it type guard:
// const validate = ajv.compile<MyData>(schema)

const data: any = {foo: 1}

if (validate(data)) {
  // data is MyData here
  console.log(data.foo)
} else {
  // The type cast is needed to allow user-defined keywords and errors
  // You can extend this type to include your error types as needed.
  for (const err of validate.errors as DefinedError[]) {
    switch (err.keyword) {
      case "minimum":
        // err type is narrowed here to have "minimum" error params properties
        console.log(err.params.limit)
        break
      // ...
    }
  }
}

Typescript compiler error messages

TSError: ⨯ Unable to compile TypeScript:
index.ts:11:5 - error TS2322: Type '{ type: "number"; minimum: number; }' is not assignable to type '{ $ref: string; } | ({ type: "number" | "integer"; minimum?: number; maximum?: number; exclusiveMinimum?: number; exclusiveMaximum?: number; multipleOf?: number; format?: string; } & { [keyword: string]: any; ... 10 more ...; not?: Partial<...>; } & { ...; })'.
  Type '{ type: "number"; minimum: number; }' is not assignable to type '{ type: "number" | "integer"; minimum?: number; maximum?: number; exclusiveMinimum?: number; exclusiveMaximum?: number; multipleOf?: number; format?: string; } & { [keyword: string]: any; $id?: string; $ref?: string; ... 8 more ...; not?: Partial<...>; } & { ...; }'.
    Property 'nullable' is missing in type '{ type: "number"; minimum: number; }' but required in type '{ nullable: true; const?: never; enum?: number[]; default?: number; }'.

11     foo: {type: "number", minimum: 0},
       ~~~

  node_modules/ajv/dist/types/json-schema.d.ts:85:5
    85     nullable: true;
           ~~~~~~~~
    'nullable' is declared here.
  index.ts:5:16
    5 type MyData = {foo: number}
                     ~~~
    The expected type comes from property 'foo' which is declared here on type 'PropertiesSchema<MyData>'
index.ts:13:14 - error TS2322: Type 'string' is not assignable to type 'never'.

13   required: ["foo"],
                ~~~~~

    at createTSError (/usr/local/lib/node_modules/ts-node-fm/src/index.ts:226:12)
    at getOutput (/usr/local/lib/node_modules/ts-node-fm/src/index.ts:335:40)
    at Object.compile (/usr/local/lib/node_modules/ts-node-fm/src/index.ts:368:11)
    at startRepl (/usr/local/lib/node_modules/ts-node-fm/src/bin.ts:147:28)
    at Object.<anonymous> (/usr/local/lib/node_modules/ts-node-fm/src/bin.ts:66:1)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)

Describe the change that should be made to address the issue?

Type for nullable may be optional and/or accept false.

Type for required should accept strings.

Are you going to resolve the issue?

With more insight about the issue, I may be able to submit a PR, changing the types or changing the documentation if I'm not using AJV correctly.

[epoberezkin]

I think the issue can be that you don’t have strictNullChecks enabled. Without this option all properties are probably optional, and JSONSchemaType does not allow using optional properties in “required”

Not sure what is the right solution here.

[epoberezkin]

If that’s the case, I am inclined to categorise it as “limitation” and add to the docs that using this type requires option strictNullChecks... Need to test it first.

[GabDug]

Adding strictNullChecks: true to the tsconfig.json indeed did the trick!

Fortunately, this option didn't mess with the rest of my project.

Thank you for your accurate and quick reply!

[linusnorton]

I also hit this issue. It was an easy fix but I think it might be worth adding a note in the README because it wasn't obvious from the TS error message that it was the problem.

[epoberezkin]

added to docs