[FIX][typescript-fetch] Fix duplicate imports for models with a discriminator (#19195)

* add new typescript-fetch self import issue sample

* add a new typescript-fetch self import issue config

* generate sample

* regenerate sample

* Prevent duplicate typescript imports

* Update generated samples

* Filter imports instead of discriminators mapped models

* Update generated samples

---------

Co-authored-by: GeroSchaarmann <Gero.S97@web.de>

bin/configs/typescript-fetch-self-import-issue.yaml

@@ -0,0 +1,7 @@
+generatorName: typescript-fetch
+outputDir: samples/client/others/typescript-fetch/self-import-issue
+inputSpec: modules/openapi-generator/src/test/resources/3_0/typescript-fetch/self-import-issue.yaml
+templateDir: modules/openapi-generator/src/main/resources/typescript-fetch
+additionalProperties:
+  typescriptThreePlus: "true"
+  legacyDiscriminatorBehavior: "false"

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java

@@ -41,6 +41,7 @@ import java.io.File;
 import java.util.*;
 import java.util.stream.Collectors;
 
+import static java.util.Objects.nonNull;
 import static org.openapitools.codegen.utils.CamelizeOption.LOWERCASE_FIRST_LETTER;
 import static org.openapitools.codegen.utils.StringUtils.*;
 
@@ -387,8 +388,32 @@ public class TypeScriptFetchClientCodegen extends AbstractTypeScriptClientCodege
         for (ModelsMap entry : result.values()) {
             for (ModelMap model : entry.getModels()) {
                 ExtendedCodegenModel codegenModel = (ExtendedCodegenModel) model.getModel();
-                model.put("hasImports", codegenModel.imports.size() > 0);
+                boolean importsPresent = !codegenModel.imports.isEmpty();
+
+                // When legacyDiscriminatorBehaviour = false, DefaultCodegen will add the mapped models of the
+                // discriminator to codegenModel.imports, causing us to duplicate the import if we don't remove them
+                CodegenDiscriminator discriminator = codegenModel.discriminator;
+                boolean mappedDiscriminatorModelsPresent = nonNull(discriminator)
+                        && nonNull(discriminator.getMappedModels());
+                if (importsPresent && mappedDiscriminatorModelsPresent) {
+                    Set<String> mappedDiscriminatorModelNames = discriminator.getMappedModels()
+                            .stream()
+                            .map(CodegenDiscriminator.MappedModel::getModelName)
+                            .collect(Collectors.toSet());
+                    Set<String> filteredImports = codegenModel.imports
+                            .stream()
+                            .filter(modelImport ->
+                                    !mappedDiscriminatorModelNames.contains(modelImport))
+                            .collect(Collectors.toSet());
+
+                    codegenModel.imports.clear();
+                    codegenModel.imports.addAll(filteredImports);
+                }
+
+                model.put("hasImports", importsPresent);
                 model.put("tsImports", toTsImports(codegenModel, parseImports(codegenModel)));
+
+
                 allModels.add(codegenModel);
                 if (codegenModel.isEntity) {
                     entityModelClassnames.add(codegenModel.classname);

modules/openapi-generator/src/test/resources/3_0/typescript-fetch/self-import-issue.yaml

@@ -0,0 +1,34 @@
+openapi: "3.0.1"
+info:
+  title: Example
+  version: "1"
+paths: {}
+components:
+  schemas:
+    BranchDto:
+      type: object
+      properties:
+        name:
+          type: string
+    AbstractUserDto:
+      type: object
+      properties:
+        username:
+          type: string
+        branch:
+          "$ref": "#/components/schemas/BranchDto"
+        type:
+          type: string
+      discriminator:
+        propertyName: type
+        mapping:
+          internal-authenticated: "#/components/schemas/InternalAuthenticatedUserDto"
+          remote-authenticated: "#/components/schemas/RemoteAuthenticatedUserDto"
+    InternalAuthenticatedUserDto:
+      type: object
+      allOf:
+        - "$ref": "#/components/schemas/AbstractUserDto"
+    RemoteAuthenticatedUserDto:
+      type: object
+      allOf:
+        - "$ref": "#/components/schemas/AbstractUserDto"

samples/client/others/typescript-fetch/self-import-issue/.openapi-generator-ignore

@@ -0,0 +1,23 @@
+# OpenAPI Generator Ignore
+# Generated by openapi-generator https://github.com/openapitools/openapi-generator
+
+# Use this file to prevent files from being overwritten by the generator.
+# The patterns follow closely to .gitignore or .dockerignore.
+
+# As an example, the C# client generator defines ApiClient.cs.
+# You can make changes and tell OpenAPI Generator to ignore just this file by uncommenting the following line:
+#ApiClient.cs
+
+# You can match any string of characters against a directory, file or extension with a single asterisk (*):
+#foo/*/qux
+# The above matches foo/bar/qux and foo/baz/qux, but not foo/bar/baz/qux
+
+# You can recursively match patterns against a directory, file or extension with a double asterisk (**):
+#foo/**/qux
+# This matches foo/bar/qux, foo/baz/qux, and foo/bar/baz/qux
+
+# You can also negate patterns with an exclamation (!).
+# For example, you can ignore all files in a docs folder with the file extension .md:
+#docs/*.md
+# Then explicitly reverse the ignore rule for a single file:
+#!docs/README.md

samples/client/others/typescript-fetch/self-import-issue/.openapi-generator/FILES

@@ -0,0 +1,7 @@
+index.ts
+models/AbstractUserDto.ts
+models/BranchDto.ts
+models/InternalAuthenticatedUserDto.ts
+models/RemoteAuthenticatedUserDto.ts
+models/index.ts
+runtime.ts

samples/client/others/typescript-fetch/self-import-issue/.openapi-generator/VERSION

@@ -0,0 +1 @@
+7.8.0-SNAPSHOT

samples/client/others/typescript-fetch/self-import-issue/index.ts

@@ -0,0 +1,4 @@
+/* tslint:disable */
+/* eslint-disable */
+export * from './runtime';
+export * from './models/index';

samples/client/others/typescript-fetch/self-import-issue/models/AbstractUserDto.ts

@@ -0,0 +1,93 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Example
+ * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+ *
+ * The version of the OpenAPI document: 1
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+import { mapValues } from '../runtime';
+import type { BranchDto } from './BranchDto';
+import {
+    BranchDtoFromJSON,
+    BranchDtoFromJSONTyped,
+    BranchDtoToJSON,
+} from './BranchDto';
+
+import { InternalAuthenticatedUserDtoFromJSONTyped } from './InternalAuthenticatedUserDto';
+import { RemoteAuthenticatedUserDtoFromJSONTyped } from './RemoteAuthenticatedUserDto';
+/**
+ * 
+ * @export
+ * @interface AbstractUserDto
+ */
+export interface AbstractUserDto {
+    /**
+     * 
+     * @type {string}
+     * @memberof AbstractUserDto
+     */
+    username?: string;
+    /**
+     * 
+     * @type {BranchDto}
+     * @memberof AbstractUserDto
+     */
+    branch?: BranchDto;
+    /**
+     * 
+     * @type {string}
+     * @memberof AbstractUserDto
+     */
+    type?: string;
+}
+
+/**
+ * Check if a given object implements the AbstractUserDto interface.
+ */
+export function instanceOfAbstractUserDto(value: object): value is AbstractUserDto {
+    return true;
+}
+
+export function AbstractUserDtoFromJSON(json: any): AbstractUserDto {
+    return AbstractUserDtoFromJSONTyped(json, false);
+}
+
+export function AbstractUserDtoFromJSONTyped(json: any, ignoreDiscriminator: boolean): AbstractUserDto {
+    if (json == null) {
+        return json;
+    }
+    if (!ignoreDiscriminator) {
+        if (json['type'] === 'internal-authenticated') {
+            return InternalAuthenticatedUserDtoFromJSONTyped(json, true);
+        }
+        if (json['type'] === 'remote-authenticated') {
+            return RemoteAuthenticatedUserDtoFromJSONTyped(json, true);
+        }
+    }
+    return {
+        
+        'username': json['username'] == null ? undefined : json['username'],
+        'branch': json['branch'] == null ? undefined : BranchDtoFromJSON(json['branch']),
+        'type': json['type'] == null ? undefined : json['type'],
+    };
+}
+
+export function AbstractUserDtoToJSON(value?: AbstractUserDto | null): any {
+    if (value == null) {
+        return value;
+    }
+    return {
+        
+        'username': value['username'],
+        'branch': BranchDtoToJSON(value['branch']),
+        'type': value['type'],
+    };
+}
+

samples/client/others/typescript-fetch/self-import-issue/models/BranchDto.ts

@@ -0,0 +1,60 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Example
+ * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+ *
+ * The version of the OpenAPI document: 1
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+import { mapValues } from '../runtime';
+/**
+ * 
+ * @export
+ * @interface BranchDto
+ */
+export interface BranchDto {
+    /**
+     * 
+     * @type {string}
+     * @memberof BranchDto
+     */
+    name?: string;
+}
+
+/**
+ * Check if a given object implements the BranchDto interface.
+ */
+export function instanceOfBranchDto(value: object): value is BranchDto {
+    return true;
+}
+
+export function BranchDtoFromJSON(json: any): BranchDto {
+    return BranchDtoFromJSONTyped(json, false);
+}
+
+export function BranchDtoFromJSONTyped(json: any, ignoreDiscriminator: boolean): BranchDto {
+    if (json == null) {
+        return json;
+    }
+    return {
+        
+        'name': json['name'] == null ? undefined : json['name'],
+    };
+}
+
+export function BranchDtoToJSON(value?: BranchDto | null): any {
+    if (value == null) {
+        return value;
+    }
+    return {
+        
+        'name': value['name'],
+    };
+}
+

samples/client/others/typescript-fetch/self-import-issue/models/InternalAuthenticatedUserDto.ts

@@ -0,0 +1,55 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Example
+ * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+ *
+ * The version of the OpenAPI document: 1
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+import { mapValues } from '../runtime';
+import type { BranchDto } from './BranchDto';
+import {
+    BranchDtoFromJSON,
+    BranchDtoFromJSONTyped,
+    BranchDtoToJSON,
+} from './BranchDto';
+import type { AbstractUserDto } from './AbstractUserDto';
+import {
+    AbstractUserDtoFromJSON,
+    AbstractUserDtoFromJSONTyped,
+    AbstractUserDtoToJSON,
+} from './AbstractUserDto';
+
+/**
+ * 
+ * @export
+ * @interface InternalAuthenticatedUserDto
+ */
+export interface InternalAuthenticatedUserDto extends AbstractUserDto {
+}
+
+/**
+ * Check if a given object implements the InternalAuthenticatedUserDto interface.
+ */
+export function instanceOfInternalAuthenticatedUserDto(value: object): value is InternalAuthenticatedUserDto {
+    return true;
+}
+
+export function InternalAuthenticatedUserDtoFromJSON(json: any): InternalAuthenticatedUserDto {
+    return InternalAuthenticatedUserDtoFromJSONTyped(json, false);
+}
+
+export function InternalAuthenticatedUserDtoFromJSONTyped(json: any, ignoreDiscriminator: boolean): InternalAuthenticatedUserDto {
+    return json;
+}
+
+export function InternalAuthenticatedUserDtoToJSON(value?: InternalAuthenticatedUserDto | null): any {
+    return value;
+}
+

samples/client/others/typescript-fetch/self-import-issue/models/RemoteAuthenticatedUserDto.ts

@@ -0,0 +1,55 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Example
+ * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+ *
+ * The version of the OpenAPI document: 1
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+import { mapValues } from '../runtime';
+import type { BranchDto } from './BranchDto';
+import {
+    BranchDtoFromJSON,
+    BranchDtoFromJSONTyped,
+    BranchDtoToJSON,
+} from './BranchDto';
+import type { AbstractUserDto } from './AbstractUserDto';
+import {
+    AbstractUserDtoFromJSON,
+    AbstractUserDtoFromJSONTyped,
+    AbstractUserDtoToJSON,
+} from './AbstractUserDto';
+
+/**
+ * 
+ * @export
+ * @interface RemoteAuthenticatedUserDto
+ */
+export interface RemoteAuthenticatedUserDto extends AbstractUserDto {
+}
+
+/**
+ * Check if a given object implements the RemoteAuthenticatedUserDto interface.
+ */
+export function instanceOfRemoteAuthenticatedUserDto(value: object): value is RemoteAuthenticatedUserDto {
+    return true;
+}
+
+export function RemoteAuthenticatedUserDtoFromJSON(json: any): RemoteAuthenticatedUserDto {
+    return RemoteAuthenticatedUserDtoFromJSONTyped(json, false);
+}
+
+export function RemoteAuthenticatedUserDtoFromJSONTyped(json: any, ignoreDiscriminator: boolean): RemoteAuthenticatedUserDto {
+    return json;
+}
+
+export function RemoteAuthenticatedUserDtoToJSON(value?: RemoteAuthenticatedUserDto | null): any {
+    return value;
+}
+

samples/client/others/typescript-fetch/self-import-issue/models/index.ts

@@ -0,0 +1,6 @@
+/* tslint:disable */
+/* eslint-disable */
+export * from './AbstractUserDto';
+export * from './BranchDto';
+export * from './InternalAuthenticatedUserDto';
+export * from './RemoteAuthenticatedUserDto';

samples/client/others/typescript-fetch/self-import-issue/runtime.ts

@@ -0,0 +1,426 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * Example
+ * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+ *
+ * The version of the OpenAPI document: 1
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+export const BASE_PATH = "http://localhost".replace(/\/+$/, "");
+
+export interface ConfigurationParameters {
+    basePath?: string; // override base path
+    fetchApi?: FetchAPI; // override for fetch implementation
+    middleware?: Middleware[]; // middleware to apply before/after fetch requests
+    queryParamsStringify?: (params: HTTPQuery) => string; // stringify function for query strings
+    username?: string; // parameter for basic security
+    password?: string; // parameter for basic security
+    apiKey?: string | Promise<string> | ((name: string) => string | Promise<string>); // parameter for apiKey security
+    accessToken?: string | Promise<string> | ((name?: string, scopes?: string[]) => string | Promise<string>); // parameter for oauth2 security
+    headers?: HTTPHeaders; //header params we want to use on every request
+    credentials?: RequestCredentials; //value for the credentials param we want to use on each request
+}
+
+export class Configuration {
+    constructor(private configuration: ConfigurationParameters = {}) {}
+
+    set config(configuration: Configuration) {
+        this.configuration = configuration;
+    }
+
+    get basePath(): string {
+        return this.configuration.basePath != null ? this.configuration.basePath : BASE_PATH;
+    }
+
+    get fetchApi(): FetchAPI | undefined {
+        return this.configuration.fetchApi;
+    }
+
+    get middleware(): Middleware[] {
+        return this.configuration.middleware || [];
+    }
+
+    get queryParamsStringify(): (params: HTTPQuery) => string {
+        return this.configuration.queryParamsStringify || querystring;
+    }
+
+    get username(): string | undefined {
+        return this.configuration.username;
+    }
+
+    get password(): string | undefined {
+        return this.configuration.password;
+    }
+
+    get apiKey(): ((name: string) => string | Promise<string>) | undefined {
+        const apiKey = this.configuration.apiKey;
+        if (apiKey) {
+            return typeof apiKey === 'function' ? apiKey : () => apiKey;
+        }
+        return undefined;
+    }
+
+    get accessToken(): ((name?: string, scopes?: string[]) => string | Promise<string>) | undefined {
+        const accessToken = this.configuration.accessToken;
+        if (accessToken) {
+            return typeof accessToken === 'function' ? accessToken : async () => accessToken;
+        }
+        return undefined;
+    }
+
+    get headers(): HTTPHeaders | undefined {
+        return this.configuration.headers;
+    }
+
+    get credentials(): RequestCredentials | undefined {
+        return this.configuration.credentials;
+    }
+}
+
+export const DefaultConfig = new Configuration();
+
+/**
+ * This is the base class for all generated API classes.
+ */
+export class BaseAPI {
+
+    private static readonly jsonRegex = new RegExp('^(:?application\/json|[^;/ \t]+\/[^;/ \t]+[+]json)[ \t]*(:?;.*)?$', 'i');
+    private middleware: Middleware[];
+
+    constructor(protected configuration = DefaultConfig) {
+        this.middleware = configuration.middleware;
+    }
+
+    withMiddleware<T extends BaseAPI>(this: T, ...middlewares: Middleware[]) {
+        const next = this.clone<T>();
+        next.middleware = next.middleware.concat(...middlewares);
+        return next;
+    }
+
+    withPreMiddleware<T extends BaseAPI>(this: T, ...preMiddlewares: Array<Middleware['pre']>) {
+        const middlewares = preMiddlewares.map((pre) => ({ pre }));
+        return this.withMiddleware<T>(...middlewares);
+    }
+
+    withPostMiddleware<T extends BaseAPI>(this: T, ...postMiddlewares: Array<Middleware['post']>) {
+        const middlewares = postMiddlewares.map((post) => ({ post }));
+        return this.withMiddleware<T>(...middlewares);
+    }
+
+    /**
+     * Check if the given MIME is a JSON MIME.
+     * JSON MIME examples:
+     *   application/json
+     *   application/json; charset=UTF8
+     *   APPLICATION/JSON
+     *   application/vnd.company+json
+     * @param mime - MIME (Multipurpose Internet Mail Extensions)
+     * @return True if the given MIME is JSON, false otherwise.
+     */
+    protected isJsonMime(mime: string | null | undefined): boolean {
+        if (!mime) {
+            return false;
+        }
+        return BaseAPI.jsonRegex.test(mime);
+    }
+
+    protected async request(context: RequestOpts, initOverrides?: RequestInit | InitOverrideFunction): Promise<Response> {
+        const { url, init } = await this.createFetchParams(context, initOverrides);
+        const response = await this.fetchApi(url, init);
+        if (response && (response.status >= 200 && response.status < 300)) {
+            return response;
+        }
+        throw new ResponseError(response, 'Response returned an error code');
+    }
+
+    private async createFetchParams(context: RequestOpts, initOverrides?: RequestInit | InitOverrideFunction) {
+        let url = this.configuration.basePath + context.path;
+        if (context.query !== undefined && Object.keys(context.query).length !== 0) {
+            // only add the querystring to the URL if there are query parameters.
+            // this is done to avoid urls ending with a "?" character which buggy webservers
+            // do not handle correctly sometimes.
+            url += '?' + this.configuration.queryParamsStringify(context.query);
+        }
+
+        const headers = Object.assign({}, this.configuration.headers, context.headers);
+        Object.keys(headers).forEach(key => headers[key] === undefined ? delete headers[key] : {});
+
+        const initOverrideFn =
+            typeof initOverrides === "function"
+                ? initOverrides
+                : async () => initOverrides;
+
+        const initParams = {
+            method: context.method,
+            headers,
+            body: context.body,
+            credentials: this.configuration.credentials,
+        };
+
+        const overriddenInit: RequestInit = {
+            ...initParams,
+            ...(await initOverrideFn({
+                init: initParams,
+                context,
+            }))
+        };
+
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
+        const init: RequestInit = {
+            ...overriddenInit,
+            body
+        };
+
+        return { url, init };
+    }
+
+    private fetchApi = async (url: string, init: RequestInit) => {
+        let fetchParams = { url, init };
+        for (const middleware of this.middleware) {
+            if (middleware.pre) {
+                fetchParams = await middleware.pre({
+                    fetch: this.fetchApi,
+                    ...fetchParams,
+                }) || fetchParams;
+            }
+        }
+        let response: Response | undefined = undefined;
+        try {
+            response = await (this.configuration.fetchApi || fetch)(fetchParams.url, fetchParams.init);
+        } catch (e) {
+            for (const middleware of this.middleware) {
+                if (middleware.onError) {
+                    response = await middleware.onError({
+                        fetch: this.fetchApi,
+                        url: fetchParams.url,
+                        init: fetchParams.init,
+                        error: e,
+                        response: response ? response.clone() : undefined,
+                    }) || response;
+                }
+            }
+            if (response === undefined) {
+              if (e instanceof Error) {
+                throw new FetchError(e, 'The request failed and the interceptors did not return an alternative response');
+              } else {
+                throw e;
+              }
+            }
+        }
+        for (const middleware of this.middleware) {
+            if (middleware.post) {
+                response = await middleware.post({
+                    fetch: this.fetchApi,
+                    url: fetchParams.url,
+                    init: fetchParams.init,
+                    response: response.clone(),
+                }) || response;
+            }
+        }
+        return response;
+    }
+
+    /**
+     * Create a shallow clone of `this` by constructing a new instance
+     * and then shallow cloning data members.
+     */
+    private clone<T extends BaseAPI>(this: T): T {
+        const constructor = this.constructor as any;
+        const next = new constructor(this.configuration);
+        next.middleware = this.middleware.slice();
+        return next;
+    }
+};
+
+function isBlob(value: any): value is Blob {
+    return typeof Blob !== 'undefined' && value instanceof Blob;
+}
+
+function isFormData(value: any): value is FormData {
+    return typeof FormData !== "undefined" && value instanceof FormData;
+}
+
+export class ResponseError extends Error {
+    override name: "ResponseError" = "ResponseError";
+    constructor(public response: Response, msg?: string) {
+        super(msg);
+    }
+}
+
+export class FetchError extends Error {
+    override name: "FetchError" = "FetchError";
+    constructor(public cause: Error, msg?: string) {
+        super(msg);
+    }
+}
+
+export class RequiredError extends Error {
+    override name: "RequiredError" = "RequiredError";
+    constructor(public field: string, msg?: string) {
+        super(msg);
+    }
+}
+
+export const COLLECTION_FORMATS = {
+    csv: ",",
+    ssv: " ",
+    tsv: "\t",
+    pipes: "|",
+};
+
+export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
+
+export type Json = any;
+export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
+export type HTTPHeaders = { [key: string]: string };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
+export type HTTPBody = Json | FormData | URLSearchParams;
+export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody };
+export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
+
+export type InitOverrideFunction = (requestContext: { init: HTTPRequestInit, context: RequestOpts }) => Promise<RequestInit>
+
+export interface FetchParams {
+    url: string;
+    init: RequestInit;
+}
+
+export interface RequestOpts {
+    path: string;
+    method: HTTPMethod;
+    headers: HTTPHeaders;
+    query?: HTTPQuery;
+    body?: HTTPBody;
+}
+
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
+    return Object.keys(params)
+        .map(key => querystringSingleKey(key, params[key], prefix))
+        .filter(part => part.length > 0)
+        .join('&');
+}
+
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
+export function mapValues(data: any, fn: (item: any) => any) {
+  return Object.keys(data).reduce(
+    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
+    {}
+  );
+}
+
+export function canConsumeForm(consumes: Consume[]): boolean {
+    for (const consume of consumes) {
+        if ('multipart/form-data' === consume.contentType) {
+            return true;
+        }
+    }
+    return false;
+}
+
+export interface Consume {
+    contentType: string;
+}
+
+export interface RequestContext {
+    fetch: FetchAPI;
+    url: string;
+    init: RequestInit;
+}
+
+export interface ResponseContext {
+    fetch: FetchAPI;
+    url: string;
+    init: RequestInit;
+    response: Response;
+}
+
+export interface ErrorContext {
+    fetch: FetchAPI;
+    url: string;
+    init: RequestInit;
+    error: unknown;
+    response?: Response;
+}
+
+export interface Middleware {
+    pre?(context: RequestContext): Promise<FetchParams | void>;
+    post?(context: ResponseContext): Promise<Response | void>;
+    onError?(context: ErrorContext): Promise<Response | void>;
+}
+
+export interface ApiResponse<T> {
+    raw: Response;
+    value(): Promise<T>;
+}
+
+export interface ResponseTransformer<T> {
+    (json: any): T;
+}
+
+export class JSONApiResponse<T> {
+    constructor(public raw: Response, private transformer: ResponseTransformer<T> = (jsonValue: any) => jsonValue) {}
+
+    async value(): Promise<T> {
+        return this.transformer(await this.raw.json());
+    }
+}
+
+export class VoidApiResponse {
+    constructor(public raw: Response) {}
+
+    async value(): Promise<void> {
+        return undefined;
+    }
+}
+
+export class BlobApiResponse {
+    constructor(public raw: Response) {}
+
+    async value(): Promise<Blob> {
+        return await this.raw.blob();
+    };
+}
+
+export class TextApiResponse {
+    constructor(public raw: Response) {}
+
+    async value(): Promise<string> {
+        return await this.raw.text();
+    };
+}