Fixing Angular v18 with Storybook v8 TypeScript Errors: 'ArgsStoryFn' Type Mismatch Problem

Overcoming Type Errors with EventEmitter in Storybook and Angular

TypeScript, Angular, and Storybook are powerful tools for creating component-driven design, but they can sometimes collide in unexpected ways, especially when TypeScript types get complicated. Recently, I encountered a baffling type error while working with Storybook v8.3.4 and Angular v18.2.6. 😕

The issue cropped up when I added an EventEmitter to a Storybook story for an Angular component. Although the EventEmitter was essential to the component's behavior, Storybook threw a type error, making it impossible to run the story smoothly. It was a frustrating hurdle, as the error message was far from helpful, mentioning a mismatch with 'ArgsStoryFn' and an incomprehensible type hierarchy.

Removing the EventEmitter resolved the error, but obviously, that wasn’t a feasible solution. After experimenting, I discovered a temporary workaround by changing the StoryObj type to 'any.' However, this solution felt clumsy, and I wanted to understand the root of the issue. 🤔

In this article, we’ll explore why this type mismatch occurs and walk through ways to troubleshoot it effectively. We’ll also cover some coding tips to help you avoid similar errors when working with Storybook and Angular components using TypeScript.

Understanding Storybook TypeScript Solutions for Angular Component Issues

The scripts created above address a specific issue with EventEmitter types in Storybook when working with Angular and TypeScript. This problem often arises when we include EventEmitter as an @Output() in Angular components and then attempt to display them in Storybook, a tool for building UI components. The type mismatch error occurs because Storybook’s typing system, particularly the ArgsStoryFn type, conflicts with Angular’s types. The first solution uses TypeScript’s Partial type, allowing us to define arguments for the render function without requiring all component properties to be included. By using Partial, Storybook can handle props more flexibly, especially for custom events like EventEmitter. For instance, if I want a button component that emits a click event, using Partial helps avoid errors even if props aren’t fully typed initially. 🎉

The second solution introduces a helper function, handleArgs, to handle properties dynamically before passing them to Storybook. This approach ensures that only properties defined in the story (like EventEmitter in this case) are passed, preventing any type conflict from undefined or incompatible types. This helper function is also valuable when handling complex components with many nested or optional properties, as it gives developers a single point to verify and adjust arguments for Storybook without modifying the component itself. The helper function creates a clean and efficient bridge between Angular and Storybook, showing how flexible solutions can simplify component integration.

In the third approach, we use TypeScript’s Omit type to exclude certain properties, like EventEmitter, that don’t directly work with Storybook’s default typing. By omitting incompatible properties, we can define custom replacements or add the property conditionally, as we did by checking if the EventEmitter is present or not. This approach is highly beneficial for large-scale projects where properties vary widely across components, as we can selectively exclude or customize properties without affecting the component’s functionality. For instance, this is useful when displaying a modal component in Storybook without initializing certain event triggers, making it easier to focus on visual elements without worrying about type conflicts.

Lastly, the unit tests are essential to verify each solution’s robustness. Unit tests using Jest’s expect function confirm that EventEmitter properties are correctly assigned and functional, making sure Storybook stories work as intended and components render without error. These tests are also great for preventing future issues, especially as your team adds or updates components. Tests, for instance, can confirm a custom dropdown component’s behavior, checking that the component triggers specific events or displays options accurately, giving developers confidence in the component’s integrity. By using these modular solutions and thorough testing, you can manage complex UI interactions smoothly, ensuring a seamless experience in both development and testing environments. 🚀

import { Meta, StoryObj } from '@storybook/angular';
import { EventEmitter } from '@angular/core';
import MyComponent from './my-component.component';
// Set up the meta configuration for Storybook
const meta: Meta<MyComponent> = {
  title: 'MyComponent',
  component: MyComponent
};
export default meta;
// Define Story type using MyComponent while maintaining types
type Story = StoryObj<Meta<MyComponent>>;
// Approach: Wrapper function to handle EventEmitter without type errors
export const Basic: Story = {
  render: (args: Partial<MyComponent>) => ({
    props: {
      ...args,
      someEvent: new EventEmitter<any>()
    }
  }),
  args: {}
};
// Unit Test to verify the EventEmitter renders correctly in Storybook
describe('MyComponent Story', () => {
  it('should render without type errors', () => {
    const emitter = new EventEmitter<any>();
    expect(emitter.observers).toBeDefined();
  });
});

import { Meta, StoryObj } from '@storybook/angular';
import MyComponent from './my-component.component';
import { EventEmitter } from '@angular/core';
// Set up Storybook metadata for the component
const meta: Meta<MyComponent> = {
  title: 'MyComponent',
  component: MyComponent
};
export default meta;
// Wrapper function for Story args handling
function handleArgs(args: Partial<MyComponent>): Partial<MyComponent> {
  return { ...args, someEvent: new EventEmitter<any>() };
}
// Define story with helper function
export const Basic: StoryObj<Meta<MyComponent>> = {
  render: (args) => ({
    props: handleArgs(args)
  }),
  args: {}
};
// Unit test for the EventEmitter wrapper function
describe('handleArgs function', () => {
  it('should attach an EventEmitter to args', () => {
    const result = handleArgs({});
    expect(result.someEvent).toBeInstanceOf(EventEmitter);
  });
});

import { Meta, StoryObj } from '@storybook/angular';
import { EventEmitter } from '@angular/core';
import MyComponent from './my-component.component';
// Define a custom type to match Storybook expectations
type MyComponentArgs = Omit<MyComponent, 'someEvent'> & {
  someEvent?: EventEmitter<any>;
};
// Set up Storybook meta
const meta: Meta<MyComponent> = {
  title: 'MyComponent',
  component: MyComponent
};
export default meta;
// Define the story using custom argument type
export const Basic: StoryObj<Meta<MyComponentArgs>> = {
  render: (args: MyComponentArgs) => ({
    props: { ...args, someEvent: args.someEvent || new EventEmitter<any>() }
  }),
  args: {}
};
// Test to verify custom types and event behavior
describe('MyComponent with Custom Types', () => {
  it('should handle MyComponentArgs without errors', () => {
    const event = new EventEmitter<any>();
    const result = { ...event };
    expect(result).toBeDefined();
  });
});

Delving Into TypeScript Compatibility with Storybook and Angular Components

In TypeScript projects involving Storybook and Angular, creating component stories becomes tricky when EventEmitters are involved. While Storybook provides an efficient platform for UI development, integrating it with Angular’s complex typings can present unique challenges. Type errors frequently occur when using Angular’s @Output() EventEmitters in stories, as the TypeScript types between Angular and Storybook don’t always align. This issue is amplified in TypeScript, where Storybook’s ArgsStoryFn type may expect props that differ from Angular’s requirements. Handling these types effectively often requires strategies like custom types or helper functions, which can help Storybook better “understand” Angular components. 🛠️

One effective approach is to customize the type compatibility using TypeScript’s advanced types, like Omit and Partial, both of which give developers control over specific type exclusions or inclusions. For instance, Omit can remove properties that cause conflicts, such as an EventEmitter, while still allowing the story to render the rest of the component accurately. Alternatively, using Partial enables developers to make each component property optional, giving Storybook more flexibility in how it handles component props. These tools are helpful for developers who frequently work with UI components that have dynamic events and are essential for balancing functionality with smooth story development.

Finally, adding comprehensive tests ensures that the custom types and workarounds function as intended across development environments. Using unit testing frameworks like Jest or Jasmine, tests can validate each type adjustment, confirm that emitted events are properly handled, and verify that the components behave as expected in Storybook. These tests prevent unexpected type errors, making development more predictable and scalable. For instance, by testing a form component’s submission event in Storybook, you can verify that user interactions trigger the EventEmitter properly, offering both development efficiency and a better user experience. 🚀