markdown 用于遍历对象和数组的属性的实用程序函数。

Posted 2021-05-06

function idx(n,e){try{return e(n)}catch(n){if(n instanceof TypeError){if(/^null | null$|^[^(]* null /.test(n))return null;if(/^undefined | undefined$|^[^(]* undefined /.test(n))return}throw n}}

/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @providesModule idx
 * 
 */

'use strict'; // eslint-disable-line strict

/**
 * Traverses properties on objects and arrays. If an intermediate property is
 * either null or undefined, it is instead returned. The purpose of this method
 * is to simplify extracting properties from a chain of maybe-typed properties.
 *
 * === EXAMPLE ===
 *
 * Consider the following type:
 *
 *   const props: {
 *     user: ?{
 *       name: string,
 *       friends: ?Array<User>,
 *     }
 *   };
 *
 * Getting to the friends of my first friend would resemble:
 *
 *   props.user &&
 *   props.user.friends &&
 *   props.user.friends[0] &&
 *   props.user.friends[0].friends
 *
 * Instead, `idx` allows us to safely write:
 *
 *   idx(props, _ => _.user.friends[0].friends)
 *
 * The second argument must be a function that returns one or more nested member
 * expressions. Any other expression has undefined behavior.
 *
 * === NOTE ===
 *
 * The code below exists for the purpose of illustrating expected behavior and
 * is not meant to be executed. The `idx` function is used in conjunction with a
 * Babel transform that replaces it with better performing code:
 *
 *   props.user == null ? props.user :
 *   props.user.friends == null ? props.user.friends :
 *   props.user.friends[0] == null ? props.user.friends[0] :
 *   props.user.friends[0].friends
 *
 * All this machinery exists due to the fact that an existential operator does
 * not currently exist in JavaScript.
 */

function idx(input, accessor) {
  try {
    return accessor(input);
  } catch (error) {
    if (error instanceof TypeError) {
      if (nullPattern.test(error)) {
        return null;
      } else if (undefinedPattern.test(error)) {
        return undefined;
      }
    }
    throw error;
  }
}

/**
 * Some actual error messages for null:
 *
 * TypeError: Cannot read property 'bar' of null
 * TypeError: Cannot convert null value to object
 * TypeError: foo is null
 * TypeError: null has no properties
 * TypeError: null is not an object (evaluating 'foo.bar')
 * TypeError: null is not an object (evaluating '(" undefined ", null).bar')
 */
var nullPattern = /^null | null$|^[^(]* null /;
var undefinedPattern = /^undefined | undefined$|^[^(]* undefined /;

idx.default = idx;
module.exports = idx;

# idx

[original repository](https://github.com/facebookincubator/idx)

`idx` is a utility function for traversing properties on objects and arrays.

If an intermediate property is either null or undefined, it is instead returned.
The purpose of this function is to simplify extracting properties from a chain
of maybe-typed properties.

This module exists as a stop-gap solution because JavaScript does not currently
support [optional chaining](https://github.com/tc39/proposal-optional-chaining).

## Usage

Consider the following type for `props`:

```javascript
type User = {
  user: ?{
    name: string,
    friends: ?Array<User>,
  }
};
```

Getting to the friends of my first friend would resemble:

```javascript
props.user &&
props.user.friends &&
props.user.friends[0] &&
props.user.friends[0].friends
```

Instead, `idx` allows us to safely write:

```javascript
idx(props, _ => _.user.friends[0].friends)
```

The second argument must be a function that returns one or more nested member
expressions. Any other expression has undefined behavior.