在《走进开源项目 - urlcat》中，对项目整体进行了分析，对如何做开源也有了进一步的了解，该篇再深入研究下 urlcat 源码。
该项目到底做了什么？
// 常规写法一
const API_URL = 'https://api.example.com/';
function getUserPosts(id, blogId, limit, offset) {
const requestUrl = `${API_URL}/users/${id}/blogs/${blogId}/posts?limit=${limit}&offset=${offset}`;
// send HTTP request
}
// 常规写法二
const API_URL = 'https://api.example.com/';
function getUserPosts(id, blogId, limit, offset) {
const escapedId = encodeURIComponent(id);
const escapedBlogId = encodeURIComponent(blogId);
const path = `/users/${escapedId}/blogs/${escapedBlogId}`;
const url = new URL(path, API_URL);
url.search = new URLSearchParams({ limit, offset });
const requestUrl = url.href;
// send HTTP request
}
// 使用 urlcat 之后的写法
const API_URL = 'https://api.example.com/';
function getUserPosts(id, limit, offset) {
const requestUrl = urlcat(API_URL, '/users/:id/posts', { id, limit, offset });
// send HTTP request
}源码共 267 行，其中注释占了近 110，代码只有 157 行。注释跟代码接近 1:1 ，接下来我们逐段分析。
第一段
import qs, { IStringifyOptions } from 'qs';
// eslint-disable-next-line @typescript-eslint/no-explicit-any
export type ParamMap = Record<string, any>;
export type UrlCatConfiguration =
Partial<Pick<IStringifyOptions, 'arrayFormat'> & { objectFormat: Partial<Pick<IStringifyOptions, 'format'>> }>该项目是在 qs 项目的基础上并使用 typescript 进行开发，其中定义了 2 个类型，有几个不太了解知识点 type 、 Recode 、Partial 和 Pick 。
interface 与 type 的区别
- 相同点：都可以描述对象或者函数，且可以使用
extends进行拓展 不同点：
type 可以声明基本类型别名，联合类型，和元组等类型，但 interface 不行
// 基本类型别名 type Name = string | number; // 联合类型 interface Common { name: string; } interface Person<T> extends Common { age: T; sex: string; } type People<T> = { age: T; sex: string; } & Common; type P1 = Person<number> | People<number>; // 元组 type P2 = [Person<number>, People<number>];跟 typeof 结合使用
const name = "小明"; type T= typeof name;
Record 的用途
Reacord 是 TypeScript 的一种工具类。
// 常规写法
interface Params {
[name: string]: any;
}
// 高级写法
type Params = Recode<string, any>Partial 的用途
将传入的属性变为可选项
interface DataModel {
name: string
age: number
address: string
}
let store: DataModel = {
name: '',
age: 0,
address: ''
}
function updateStore (
store: DataModel,
payload: Partial<DataModel>
):DataModel {
return {
...store,
...payload
}
}
store = updateStore(store, {
name: 'lpp',
age: 18
})Pick 的用途
从类型 Type 中，挑选一组属性组成一个新的类型返回。这组属性由 Keys 限定， Keys 是字符串或者字符串并集。
interface Person {
name: string
age: number
id: string
}
// 幼儿没有id
type Toddler = Pick<Person, 'name' | 'age'>第二段
/**
* Builds a URL using the base template and specified parameters.
*
* @param {String} baseTemplate a URL template that contains zero or more :params
* @param {Object} params an object with properties that correspond to the :params
* in the base template. Unused properties become query params.
*
* @returns {String} a URL with path params substituted and query params appended
*
* @example
* ```ts
* urlcat('http://api.example.com/users/:id', { id: 42, search: 'foo' })
* // -> 'http://api.example.com/users/42?search=foo
* ```
*/
export default function urlcat(baseTemplate: string, params: ParamMap): string;
/**
* Concatenates the base URL and the path specified using '/' as a separator.
* If a '/' occurs at the concatenation boundary in either parameter, it is removed.
*
* @param {String} baseUrl the first part of the URL
* @param {String} path the second part of the URL
*
* @returns {String} the result of the concatenation
*
* @example
* ```ts
* urlcat('http://api.example.com/', '/users')
* // -> 'http://api.example.com/users
* ```
*/
export default function urlcat(baseUrl: string, path: string): string;
/**
* Concatenates the base URL and the path specified using '/' as a separator.
* If a '/' occurs at the concatenation boundary in either parameter, it is removed.
* Substitutes path parameters with the properties of the @see params object and appends
* unused properties in the path as query params.
*
* @param {String} baseUrl the first part of the URL
* @param {String} path the second part of the URL
* @param {Object} params Object with properties that correspond to the :params
* in the base template. Unused properties become query params.
*
* @returns {String} URL with path params substituted and query params appended
*
* @example
* ```ts
* urlcat('http://api.example.com/', '/users/:id', { id: 42, search: 'foo' })
* // -> 'http://api.example.com/users/42?search=foo
* ```
*/
export default function urlcat(
baseUrl: string,
pathTemplate: string,
params: ParamMap
): string;
/**
* Concatenates the base URL and the path specified using '/' as a separator.
* If a '/' occurs at the concatenation boundary in either parameter, it is removed.
* Substitutes path parameters with the properties of the @see params object and appends
* unused properties in the path as query params.
*
* @param {String} baseUrl the first part of the URL
* @param {String} path the second part of the URL
* @param {Object} params Object with properties that correspond to the :params
* in the base template. Unused properties become query params.
* @param {Object} config urlcat configuration object
*
* @returns {String} URL with path params substituted and query params appended
*
* @example
* ```ts
* urlcat('http://api.example.com/', '/users/:id', { id: 42, search: 'foo' }, {objectFormat: {format: 'RFC1738'}})
* // -> 'http://api.example.com/users/42?search=foo
* ```
*/
export default function urlcat(
baseUrlOrTemplate: string,
pathTemplateOrParams: string | ParamMap,
maybeParams: ParamMap,
config: UrlCatConfiguration
): string;
export default function urlcat(
baseUrlOrTemplate: string,
pathTemplateOrParams: string | ParamMap,
maybeParams: ParamMap = {},
config: UrlCatConfiguration = {}
): string {
if (typeof pathTemplateOrParams === 'string') {
const baseUrl = baseUrlOrTemplate;
const pathTemplate = pathTemplateOrParams;
const params = maybeParams;
return urlcatImpl(pathTemplate, params, baseUrl, config);
} else {
const baseTemplate = baseUrlOrTemplate;
const params = pathTemplateOrParams;
return urlcatImpl(baseTemplate, params, undefined, config);
}
}这部分代码是利用 TypeScript 定义重载函数类型，采用连续多个重载声明 + 一个函数实现的方式来实现，其作用是为了保证在调用该函数时，函数的参数及返回值都要兼容所有的重载。
例如下图，第三个参数类型在重载函数类型中并不存在。
第三段
以下代码是核心，作者通过职责分离的方式，将核心方法代码简化。
// 核心方法
function urlcatImpl(
pathTemplate: string,
params: ParamMap,
baseUrl: string | undefined,
config: UrlCatConfiguration
) {
// 第一步 path('/users/:id/posts', { id: 1, limit: 30 }) 返回 "/users/1/posts" 和 limit: 30
const { renderedPath, remainingParams } = path(pathTemplate, params);
// 第二步 移除 Null 或者 Undefined 属性
const cleanParams = removeNullOrUndef(remainingParams);
// 第三步 {limit: 30} 转 limit=30
const renderedQuery = query(cleanParams, config);
// 第四步 拼接返回 /users/1/posts?limit=30
const pathAndQuery = join(renderedPath, '?', renderedQuery);
// 第五步 当 baseUrl 存在时，执行完整 url 拼接
return baseUrl ? joinFullUrl(renderedPath, baseUrl, pathAndQuery) : pathAndQuery;
}总结
做开源并不一定要造个更好的轮子，但可以让这个轮子变得更好。通过该项目，也发现自己在 TypeScript 方面的不足，继续学习，再接再厉~
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