Sådan tilføjes trinvist Flow til en eksisterende React-app

To transform this to use Flow we can first remove the PropTypes import and definitions. The // @flow annotation can then be added to line one.

For this component we are only going to type check the props passed in. To do so we will first create a Props type, much cleaner than defining each prop individually inline.

type Props = { strings: { [string_key: string]: string }, onClose: Function, isOpen: boolean, target: Function, };

Here the latter three types are self-explanatory. As strings is an object of strings an object as a map has been used, checking each key and value in the object received to check that their types match, without having to specify their exact string keys.

The prop-types definitions can then be removed along with its import. As defaultProps are not tied to this import they can, and should, remain. *See the closing ESLint comments for any errors reported at this point.

The component should now look like this:

Stateful Components

Stateful components follow some slightly different declarations. As this component is more complex we will also look at declaring types for some additional aspects.

As before, first take a look at the component before adding Flow.

Props and State

As in the function component we first remove the propTypes definition and import, and add the // @flow annotation.

First we will take a look at adding types for Props and State. Again we will create types for these:

type Props = { strings: { [string_key: string]: string }, hideBannerClick: Function, }; type State = { popoverIsOpen: boolean, };

and specify that the component will use them:

class Banner extends Component { constructor(props: Props) { super(props); this.state = { popoverIsOpen: false, }; ... }; ... };

Next we hit our first difference between Function and Stateful components, defaultProps. In a Function component these were declared as we are used to, in Stateful components the external Banner.defaultProps syntax is removed, and instead the defaults are declared within the class:

class Banner extends Component { static defaultProps = { strings: defaultStrings, }; constructor(props: Props) { ... // the below is removed // Banner.defaultProps = { // strings: defaultStrings, // };

Constructor declarations

stringWithPlaceholder is declared within the constructor. Here we are not looking at why it is declared there (we will assume there is good reason), but rather to see whether flow can be added without any changes to the existing code.

If run in its existing state we would encounter the error Cannot get this.stringWithPlaceholder because property stringWithPlaceholder is missing in Banner [1].

To fix this we must add a single line inside the Banner class block, just beneath and outside of the constructor:

class Banner extends Component { constructor(props: Props) { super(props); this.state = { popoverIsOpen: false, }; this.stringWithPlaceholder = ... }; stringWithPlaceholder: string; ... };

This variable is created in the constructor but not passed in as props. As we are using Flow for type checking the props passed into the constructor, it requires everything within the constructor be type checked. It is known that Flow requires this, and this can be done by specifying their type in the class block.

At this point Props and State are complete. Let’s look at some quick additional examples of type checking within this component. *See the closing ESLint comments for any errors reported at this point.

Return, Event, and Node types

togglePopover takes no arguments, so a simple example of specifying no return value can be seen:

togglePopover = (): void => { ... };

keyboardOnlyTogglePopover returns nothing, but has a single parameter. This is an event, specifically a keypress event. SyntheticKeyboardEvent is used as

keyboardOnlyTogglePopover = (e: SyntheticKeyboardEvent): void => { ... };

Popover is defined in render() and returns an instance of the ListPopover Function component we looked a previously. We can specify its return type as a React Node. However, to be able to do so, we must first import it, as it is not accessible by default. There is more than one way to import it, one of which shown below:

import React, { Component } from 'react'; import type { Node } from 'react'; ... const Popover: Node = (  document.getElementById('ListPopoverLink')} /> );

Type checking imported React components

When Prop types have been declared in a component, they can be used when using that component within another. However, if you are using an index.js to export the first component then the flow, // @flow will need to be added to the index.

For example:

// @flow import ListPopover from './ListPopover'; export default ListPopover;

Marking props as optional

A prop can be marked as optional using the prop?: type syntax, for example:

type Props = { strings: { [string_key: string]: string }, hideBannerClick?: Function, };

This is supported, but no longer recommended by Flow. Instead all props should be left as required, with no ? , even if optional, as Flow automatically detects defaultProps and marks props with a default as optional internally.

In the section below we can see how manually marking props as optional can cause conflicts with other tools in some cases.

ESLint extensions, default props, and props validation error solutions

Two additions are made to our .eslintrc. For this project specifically you can simply accept their use, or read the detail below if you see any of the three errors:

• x missing in props validation
• error defaultProp "x" defined for isRequired propType
• Cannot get strings.xxx because property xxx is missing in undefined

The rules added, with reasoning, are:

"react/default-props-match-prop-types": [ "error", { "allowRequiredDefaults": true } ]

When using objects as maps (in this case for the 'strings' prop) a missing in props validation error occurs. This is a bug and so is explicitly ignored here.

"react/default-props-match-prop-types": [ "error", { "allowRequiredDefaults": true }]

When using objects as maps complexities between ESLint, flow, and prop-types come into play.

strings is a required prop, passed as an object of strings. The flow type checks that for each entry in the object the string key is a string, and the value is a string. This is far more maintainable than having to list out the prop type of each specific key.

If the prop is marked as required in Flow then ESLint would error stating: error defaultProp "strings" defined for isRequired propType.

If the prop is manually marked as optional then Flow will error with Cannot get strings.xxx because property xxx is missing in undefined [1].

This is known and is due to refinement invalidation as JSX can transform method calls so Flow cannot be sure that xxx has not been redefined.

This leaves us with fixing the ESLint error. The rules above allows defaultProps to be defined while the Flow type is not marked as optional. Flow will understand this and convert it to optional. ESLint is marked to "allowRequiredDefaults": true, meaning that although ESLint sees the prop as required it will not error.

Final thoughts

Once over the initial hurdle of installation, Flow is fairly straightforward to use. The ability to add it incrementally definitely helps, rather than having to refactor an entire project in one go.

Hopefully the setup instructions and examples here prove useful if you are looking to try Flow out yourself.

Thanks for reading ?

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