103 lines
2.4 KiB
Markdown
103 lines
2.4 KiB
Markdown
# idx [](https://circleci.com/gh/facebookincubator/idx)
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`idx` is a utility function for traversing properties on objects and arrays.
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If an intermediate property is either null or undefined, it is instead returned.
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The purpose of this function is to simplify extracting properties from a chain
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of maybe-typed properties.
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This module exists as a stop-gap solution because JavaScript does not currently
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support [optional chaining](https://github.com/tc39/proposal-optional-chaining).
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## Usage
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Consider the following type for `props`:
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```javascript
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type User = {
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user: ?{
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name: string,
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friends: ?Array<User>,
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}
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};
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```
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Getting to the friends of my first friend would resemble:
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```javascript
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props.user &&
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props.user.friends &&
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props.user.friends[0] &&
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props.user.friends[0].friends
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```
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Instead, `idx` allows us to safely write:
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```javascript
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idx(props, _ => _.user.friends[0].friends)
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```
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The second argument must be a function that returns one or more nested member
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expressions. Any other expression has undefined behavior.
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## Flow Type
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[Flow](https://flow.org/) understands the `idx` idiom:
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```javascript
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// @flow
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import idx from 'idx';
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function getName(props: User): ?string {
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return idx(props, _ => _.user.name);
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}
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```
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## Babel Transform
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The `idx` runtime function exists for the purpose of illustrating the expected
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behavior and is not meant to be executed. The `idx` function is used in
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conjunction with a Babel plugin that replaces it with better performing code.
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This babel plugin searches for requires or imports to the `idx` module and
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replaces all its usages, so this code:
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```javascript
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import idx from 'idx';
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function getFriends() {
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return idx(props, _ => _.user.friends[0].friends)
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};
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```
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gets transformed to something like:
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```javascript
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function getFriends() {
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props.user == null ? props.user :
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props.user.friends == null ? props.user.friends :
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props.user.friends[0] == null ? props.user.friends[0] :
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return props.user.friends[0].friends
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}
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```
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(note that the original `import` gets also removed).
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It's possible to customize the name of the import/require, so code that is not
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directly requiring the `idx` npm package can also get transformed:
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```javascript
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{
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plugins: [
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["babel-plugin-idx", {
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importName: './idx',
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}]
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]
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}
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```
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## License
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`idx` is [MIT licensed](./LICENSE).
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