Using JSONSchemaType requires strictNullChecks option

Ajv v8.2.0

Straight from the example here: https://ajv.js.org/guide/typescript.html

You can see this error in the TS Playground

import Ajv, {JSONSchemaType} from "ajv"
const ajv = new Ajv()

interface MyData {
  foo: number
  bar?: string
}

const schema: JSONSchemaType<MyData> = {
  type: "object",
  properties: {
    foo: {type: "integer"},
    bar: {type: "string", nullable: "true"}
  },
  required: ["foo"],
  additionalProperties: false
}

TS Error:

Type '{ type: "object"; properties: { foo: { type: "integer"; }; bar: { type: "string"; nullable: string; }; }; required: "foo"[]; additionalProperties: false; }' is not assignable to type 'JSONSchemaType<MyData, false>'.
  Type '{ type: "object"; properties: { foo: { type: "integer"; }; bar: { type: "string"; nullable: string; }; }; required: "foo"[]; additionalProperties: false; }' is not assignable to type '{ type: "object"; additionalProperties?: boolean | JSONSchemaType<unknown, false> | undefined; unevaluatedProperties?: boolean | JSONSchemaType<unknown, false> | undefined; ... 7 more ...; maxProperties?: number | undefined; } & { ...; } & { ...; } & { ...; }'.
    Type '{ type: "object"; properties: { foo: { type: "integer"; }; bar: { type: "string"; nullable: string; }; }; required: "foo"[]; additionalProperties: false; }' is not assignable to type '{ type: "object"; additionalProperties?: boolean | JSONSchemaType<unknown, false> | undefined; unevaluatedProperties?: boolean | JSONSchemaType<unknown, false> | undefined; ... 7 more ...; maxProperties?: number | undefined; }'.
      The types of 'properties.bar' are incompatible between these types.
        Type '{ type: "string"; nullable: string; }' is not assignable to type '{ $ref: string; } | (JSONSchemaType<string | undefined, false> & { nullable: true; const?: undefined; enum?: readonly (string | null | undefined)[] | undefined; default?: string | null | undefined; })'.
          Type '{ type: "string"; nullable: string; }' is not assignable to type '{ type: "string"; } & StringKeywords & { allOf?: readonly PartialSchema<string | undefined>[] | undefined; anyOf?: readonly PartialSchema<string | undefined>[] | undefined; ... 4 more ...; not?: PartialSchema<...> | undefined; } & { ...; } & { ...; }'.
            Type '{ type: "string"; nullable: string; }' is not assignable to type '{ nullable: true; const?: undefined; enum?: readonly (string | null | undefined)[] | undefined; default?: string | null | undefined; }'.
              Types of property 'nullable' are incompatible.
                Type 'string' is not assignable to type 'true'.ts(2322)

[sean256]

@epoberezkin here you go: https://github.com/sean256/ajv-bug 2 dependencies, ajv and typescript@4.2.4

This example repo I just made reproduces the bug, has as few dependencies as possible, meets your TS version req, and has strictNullChecks on.

[kg-currenxie]

/node_modules/ajv/dist/types/json-schema.d.ts
'?' expected.  TS1005

  > 1 | declare type StrictNullChecksWrapper<Name extends string, Type> = undefined extends null ? `strictNullChecks must be true in tsconfig to use ${Name}` : Type;

We do have strictNullChecks on already. ajv 8.3.0

{
  "compilerOptions": {
    "target": "es6",
    "lib": [
      "dom",
      "dom.iterable",
      "esnext"
    ],
    "allowJs": true,
    "allowSyntheticDefaultImports": true,
    "baseUrl": "src",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "jsx": "preserve",
    "module": "esnext",
    "moduleResolution": "node",
    "noEmit": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noUnusedLocals": true,
    "resolveJsonModule": true,
    "suppressImplicitAnyIndexErrors": true,
    "skipLibCheck": true,
    "strict": true,
    "strictNullChecks": true <------------------ 
  },
  "include": [
    "src/**/types/*.d.ts",
    "src/**/*"
  ],
  "exclude": [
    "node_modules",
    "**/*.mdx",
    "build",
    "acceptance-tests",
    "webpack",
    "jest",
    "**/__tests__/*",
    "**/scripts",
    "**/setupTests.ts",
    "**/config/**/*.js",
    "**/cypress/**/*"
  ]
}