作者：Daniel Bartholomae
翻译：疯狂的技术宅
近几年 TypeScript 和 JavaScript 一直在稳步发展。我们在过去写代码时养成了一些习惯，而有些习惯却没有什么意义。以下是我们都应该改正的 10 个坏习惯。
1.不使用 strict 模式
这种习惯看起来是什么样的
没有用严格模式编写 tsconfig.json。
<pre class="json hljs language-json" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">{
"compilerOptions": {
"target": "ES2015",
"module": "commonjs"
}
}</pre>
应该怎样
只需启用 strict 模式即可：
<pre class="json hljs language-json" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">{
"compilerOptions": {
"target": "ES2015",
"module": "commonjs",
"strict": true
}
}</pre>
为什么会有这种坏习惯
在现有代码库中引入更严格的规则需要花费时间。
为什么不该这样做
更严格的规则使将来维护代码时更加容易，使你节省大量的时间。
2. 用 || 定义默认值
这种习惯看起来是什么样的
使用旧的 || 处理后备的默认值：
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createBlogPost (text: string, author: string, date?: Date) {
return {
text: text,
author: author,
date: date || new Date()
}
}</pre>
应该怎样
使用新的 ?? 运算符，或者在参数重定义默认值。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createBlogPost (text: string, author: string, date: Date = new Date())
return {
text: text,
author: author,
date: date
}
}</pre>
为什么会有这种坏习惯
?? 运算符是去年才引入的，当在长函数中使用值时，可能很难将其设置为参数默认值。
为什么不该这样做
?? 与 || 不同，?? 仅针对 null 或 undefined，并不适用于所有虚值。
3. 随意使用 any 类型
这种习惯看起来是什么样的
当你不确定结构时，可以用 any 类型。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">async function loadProducts(): Promise<Product[]> {
const response = await fetch('https://api.mysite.com/products')
const products: any = await response.json()
return products
}</pre>
应该怎样
把你代码中任何一个使用 any 的地方都改为 unknown
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">async function loadProducts(): Promise<Product[]> {
const response = await fetch('https://api.mysite.com/products')
const products: unknown = await response.json()
return products as Product[]
}</pre>
为什么会有这种坏习惯
any 是很方便的，因为它基本上禁用了所有的类型检查。通常，甚至在官方提供的类型中都使用了 any。例如，TypeScript 团队将上面例子中的 response.json() 的类型设置为 Promise <any>。
为什么不该这样做
它基本上禁用所有类型检查。任何通过 any 进来的东西将完全放弃所有类型检查。这将会使错误很难被捕获到。
4. val as SomeType
这种习惯看起来是什么样的
强行告诉编译器无法推断的类型。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">async function loadProducts(): Promise<Product[]> {
const response = await fetch('https://api.mysite.com/products')
const products: unknown = await response.json()
return products as Product[]
}</pre>
应该怎样
这正是 Type Guard 的用武之地。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function isArrayOfProducts (obj: unknown): obj is Product[] {
return Array.isArray(obj) && obj.every(isProduct)
}
function isProduct (obj: unknown): obj is Product {
return obj != null
&& typeof (obj as Product).id === 'string'
}
async function loadProducts(): Promise<Product[]> {
const response = await fetch('https://api.mysite.com/products')
const products: unknown = await response.json()
if (!isArrayOfProducts(products)) {
throw new TypeError('Received malformed products API response')
}
return products
}</pre>
为什么会有这种坏习惯
从 JavaScript 转到 TypeScript 时，现有的代码库通常会对 TypeScript 编译器无法自动推断出的类型进行假设。在这时，通过 as SomeOtherType 可以加快转换速度，而不必修改 tsconfig 中的设置。
为什么不该这样做
Type Guard 会确保所有检查都是明确的。
5. 测试中的 as any
这种习惯看起来是什么样的
编写测试时创建不完整的用例。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">interface User {
id: string
firstName: string
lastName: string
email: string
}
test('createEmailText returns text that greats the user by first name', () => {
const user: User = {
firstName: 'John'
} as any
expect(createEmailText(user)).toContain(user.firstName)
}</pre>
应该怎样
如果你需要模拟测试数据，请将模拟逻辑移到要模拟的对象旁边，并使其可重用。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">interface User {
id: string
firstName: string
lastName: string
email: string
}
class MockUser implements User {
id = 'id'
firstName = 'John'
lastName = 'Doe'
email = 'john@doe.com'
}
test('createEmailText returns text that greats the user by first name', () => {
const user = new MockUser()
expect(createEmailText(user)).toContain(user.firstName)
}</pre>
为什么会有这种坏习惯
在给尚不具备广泛测试覆盖条件的代码编写测试时，通常会存在复杂的大数据结构，但要测试的特定功能仅需要其中的一部分。短期内不必关心其他属性。
为什么不该这样做
在某些情况下，被测代码依赖于我们之前认为不重要的属性，然后需要更新针对该功能的所有测试。
6. 可选属性
这种习惯看起来是什么样的
将属性标记为可选属性，即便这些属性有时不存在。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">interface Product {
id: string
type: 'digital' | 'physical'
weightInKg?: number
sizeInMb?: number
}</pre>
应该怎样
明确哪些组合存在，哪些不存在。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">interface Product {
id: string
type: 'digital' | 'physical'
}
interface DigitalProduct extends Product {
type: 'digital'
sizeInMb: number
}
interface PhysicalProduct extends Product {
type: 'physical'
weightInKg: number
}</pre>
为什么会有这种坏习惯
将属性标记为可选而不是拆分类型更容易，并且产生的代码更少。它还需要对正在构建的产品有更深入的了解，并且如果对产品的设计有所修改，可能会限制代码的使用。
为什么不该这样做
类型系统的最大好处是可以用编译时检查代替运行时检查。通过更显式的类型，能够对可能不被注意的错误进行编译时检查，例如确保每个 DigitalProduct 都有一个 sizeInMb。
7. 用一个字母通行天下
这种习惯看起来是什么样的
用一个字母命名泛型
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function head<T> (arr: T[]): T | undefined {
return arr[0]
}</pre>
应该怎样
提供完整的描述性类型名称。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function head<Element> (arr: Element[]): Element | undefined {
return arr[0]
}</pre>
为什么会有这种坏习惯
这种写法最早来源于C++的范型库，即使是 TS 的官方文档也在用一个字母的名称。它也可以更快地输入，只需要简单的敲下一个字母 T 就可以代替写全名。
为什么不该这样做
通用类型变量也是变量，就像其他变量一样。当 IDE 开始向我们展示变量的类型细节时，我们已经慢慢放弃了用它们的名称描述来变量类型的想法。例如我们现在写代码用 const name ='Daniel'，而不是 const strName ='Daniel'。同样，一个字母的变量名通常会令人费解，因为不看声明就很难理解它们的含义。
8. 对非布尔类型的值进行布尔检查
这种习惯看起来是什么样的
通过直接将值传给 if 语句来检查是否定义了值。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (countOfNewMessages) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
应该怎样
明确检查我们所关心的状况。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (countOfNewMessages !== undefined) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
为什么会有这种坏习惯
编写简短的检测代码看起来更加简洁，使我们能够避免思考实际想要检测的内容。
为什么不该这样做
也许我们应该考虑一下实际要检查的内容。例如上面的例子以不同的方式处理 countOfNewMessages 为 0 的情况。
9. ”棒棒“运算符
这种习惯看起来是什么样的
将非布尔值转换为布尔值。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (!!countOfNewMessages) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
应该怎样
明确检查我们所关心的状况。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (countOfNewMessages !== undefined) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
为什么会有这种坏习惯
对某些人而言，理解 !! 就像是进入 JavaScript 世界的入门仪式。它看起来简短而简洁，如果你对它已经非常习惯了，就会知道它的含义。这是将任意值转换为布尔值的便捷方式。尤其是在如果虚值之间没有明确的语义界限时，例如 null、undefined 和 ''。
为什么不该这样做
与很多编码时的便捷方式一样，使用 !! 实际上是混淆了代码的真实含义。这使得新开发人员很难理解代码，无论是对一般开发人员来说还是对 JavaScript 来说都是新手。也很容易引入细微的错误。在对“非布尔类型的值”进行布尔检查时 countOfNewMessages 为 0 的问题在使用 !! 时仍然会存在。
10. != null
这种习惯看起来是什么样的
棒棒运算符的小弟 ! = null使我们能同时检查 null 和 undefined。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (countOfNewMessages != null) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
应该怎样
明确检查我们所关心的状况。
<pre class="ts hljs language-typescript" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; font-size: 14px; margin-top: 0px !important; margin-bottom: 1.25rem; overflow: auto; display: block; color: rgb(36, 41, 46); background: rgb(233, 236, 239); padding: 1rem; max-height: 35rem; line-height: 1.5; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">function createNewMessagesResponse (countOfNewMessages?: number) {
if (countOfNewMessages !== undefined) {
return You have ${countOfNewMessages} new messages
}
return 'Error: Could not retrieve number of new messages'
}</pre>
为什么会有这种坏习惯
如果你的代码在 null 和 undefined 之间没有明显的区别，那么 != null 有助于简化对这两种可能性的检查。
为什么不该这样做
尽管 null 在 JavaScript早期很麻烦，但 TypeScript 处于 strict 模式时，它却可以成为这种语言中宝贵的工具。一种常见模式是将 null 值定义为不存在的事物，将 undefined 定义为未知的事物，例如 user.firstName === null 可能意味着用户实际上没有名字，而 user.firstName === undefined 只是意味着我们尚未询问该用户（而 user.firstName === 的意思是字面意思是 '' 。
