87 lines
2.4 KiB
JavaScript
87 lines
2.4 KiB
JavaScript
/**
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* Copyright (c) 2013-present, Facebook, Inc.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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*
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*/
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'use strict'; // eslint-disable-line strict
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/**
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* Traverses properties on objects and arrays. If an intermediate property is
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* either null or undefined, it is instead returned. The purpose of this method
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* is to simplify extracting properties from a chain of maybe-typed properties.
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*
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* === EXAMPLE ===
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*
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* Consider the following type:
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*
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* const props: {
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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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*
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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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*
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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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*
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* === NOTE ===
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*
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* The code below exists for the purpose of illustrating expected behavior and
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* is not meant to be executed. The `idx` function is used in conjunction with a
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* Babel transform that replaces it with better performing code:
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*
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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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* props.user.friends[0].friends
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*
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* All this machinery exists due to the fact that an existential operator does
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* not currently exist in JavaScript.
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*/
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function idx(input, accessor) {
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try {
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return accessor(input);
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} catch (error) {
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if (error instanceof TypeError) {
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if (nullPattern.test(error)) {
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return null;
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} else if (undefinedPattern.test(error)) {
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return undefined;
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}
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}
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throw error;
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}
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}
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/**
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* Some actual error messages for null:
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*
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* TypeError: Cannot read property 'bar' of null
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* TypeError: Cannot convert null value to object
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* TypeError: foo is null
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* TypeError: null has no properties
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* TypeError: null is not an object (evaluating 'foo.bar')
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* TypeError: null is not an object (evaluating '(" undefined ", null).bar')
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*/
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var nullPattern = /^null | null$|^[^(]* null /;
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var undefinedPattern = /^undefined | undefined$|^[^(]* undefined /;
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idx.default = idx;
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module.exports = idx;
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