feat: support inferred types in conditionals

[daanboer]

Fixes #513, #1075

The infer keyword is used in conditional expressions to infer types of type parameters. It is the basis of many useful type constructs. This pull request implements support for inferred types, including inferred rest types.

The newly added InferType is a simple type that stores the name of the inferred parameter. Evaluation of inferred types happens in ConditionalTypeNodeParser, as type inference can only occur in conditional expressions. The heavy lifting is done by the isAssignableTo function. It has been updated to build a map of all inferred types. These inferred types are then passed to the sub-context when evaluating the true branch. Additionally, the RestType has been updated to not only support the ArrayType, but also TupleType and InferType.

See the added tests for examples.

[daanboer]

@domoritz please let me know if there is anything else I need to do regarding this pull request.

[akuntsch]

Hi @daanboer,

for me the schema generation of a recursive Partial does not work correctly.

type PrimitiveType =
    | string
    | number
    | boolean
    | undefined
    | null
    | symbol;

export type DeepPartial<T> = {
    [K in keyof T]?: T[K] extends Array<infer I>
        ? Array<DeepPartial<I>>
        : T[K] extends PrimitiveType ? T[K] : DeepPartial<T[K]>;
};

export type SchemaTestPartial = DeepPartial<{ a: string[] }>;

The generated schema for SchemaTestPartial is:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$ref": "#/definitions/SchemaTestPartial",
  "definitions": {
    "SchemaTestPartial": {
      "$ref": "#/definitions/DeepPartial%3Cstructure-961230428-3503-3518-961230428-3490-3519-961230428-3457-3520-961230428-0-3578%3E"
    },
    "DeepPartial<structure-961230428-3503-3518-961230428-3490-3519-961230428-3457-3520-961230428-0-3578>": {
      "type": "object",
      "properties": {
        "a": {
          "type": "array",
          "items": {
            "$ref": "#/definitions/DeepPartial%3C%3E"
          }
        }
      },
      "additionalProperties": false
    },
    "DeepPartial<>": {
      "type": "object",
      "additionalProperties": false
    }
  }
}

The array items are now of type object instead of string.

I'm not too sure what's going wrong here but I have found that I never hit your changed code in ConditionalTypeNodeParser because the following condition is always true.

ts-json-schema-generator/src/NodeParser/ConditionalTypeNodeParser.ts

Lines 22 to 25 in 1658e10

	if (checkTypeParameterName == null) {
	const result = isAssignableTo(extendsType, checkType);
	return this.childNodeParser.createType(result ? node.trueType : node.falseType, context);
	}

[daanboer]

Hi @akuntsch,

Nice find! There was indeed a bug where the inferred types where not passed to the subcontext when checkTypeParameter == null. I have fixed this in the latest commit. There is however another bug that is unrelated to this pull request. A minimal reproducible example is as follows:

type MappedType<T> = {[K in keyof T]?: T[K]};
export type StringType = MappedType<string>;

Which produces:

{
  "$ref": "#/definitions/StringType",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "definitions": {
    "StringType": {
      "additionalProperties": false,
      "type": "object"
    }
  }
}

I think you can create a new issue for this bug (or check for an existing one)!

[domoritz]

Thank you for the pr! Looks great.

[ccpu]

@daanboer The inferred types feature is very useful, thanks for the PR.

However, it does not work with template literals, so I've opened an issue (#1355 ). I'd appreciate it if you could look into it if you have a chance.

[github-actions]

🚀 PR was released in v1.1.0 🚀