Function defined in component.ts throws error "ERROR TypeError: _co.functionName is not a function" when called from storybook

[abhi9bakshi]

Describe the bug
I have defined a function to toggle text in my storybook component. On calling the function from button, application throws error
"ButtonComponent.html:1 ERROR TypeError: _co.toggleLabel is not a function".

This error occurs only if I launch my application directly to that story URL, or reload page when that URL is already loaded. If I launch app at a different story URL and then visit the button story, the toggleLabel function gets called correctly.

Inspecting the code shows that when we load the application directly to the URL, args passed to Button.stories.ts at

const Template: Story<Button> = (args: Button) => ({
  props: args,
});

are

{
    backgroundColor: undefined
    hideLabel: undefined
    label: "Button"
    onClick: ƒ actionHandler()
    primary: false
    size: "medium"
    toggleLabel: undefined
}

However, args passed when we visit the URL after application is loaded at different URL are

{
    label: "Button"
    onClick: ƒ actionHandler()
    primary: true
    size: "medium"
}

Due to which the function defined in button.component.ts gets called.

To Reproduce
You can clone the repo for error reproduction here and rpm run storybook on master branch to reproduce the issue. The bugfix branch provides a temporary fix by deleting undefined keys from args using

Object.keys(args).forEach((e) => {
    if (args[e] === undefined) delete args[e];
});

System

System:
    OS: macOS 11.2.3
    CPU: (4) x64 Intel(R) Core(TM) i5-5350U CPU @ 1.80GHz
  Binaries:
    Node: 14.16.0 - ~/.nvm/versions/node/v14.16.0/bin/node
    npm: 6.14.11 - ~/.nvm/versions/node/v14.16.0/bin/npm
  Browsers:
    Chrome: 90.0.4430.85
    Firefox: 83.0
    Safari: 14.0.3
  npmPackages:
    @storybook/addon-actions: ^6.2.8 => 6.2.8 
    @storybook/addon-essentials: ^6.2.8 => 6.2.8 
    @storybook/addon-links: ^6.2.8 => 6.2.8 
    @storybook/angular: ^6.2.8 => 6.2.8

[shilman]

closing as dupe to #14147