TypeScript 2.5

可选 catch 子句变量

¥Optional catch clause variables

得益于 @tinganho 的工作，TypeScript 2.5 实现了一项新的 ECMAScript 功能，允许用户在 catch 子句中省略变量。例如，当使用 JSON.parse 时，你可能需要使用 try/catch 封装对函数的调用，但你可能最终不会使用在输入错误时抛出的 SyntaxError。

¥Thanks to work done by @tinganho, TypeScript 2.5 implements a new ECMAScript feature that allows users to omit the variable in catch clauses. For example, when using JSON.parse you may need to wrap calls to the function with a try/catch, but you may not end up using the SyntaxError that gets thrown when input is erroneous.

ts
let input = "...";
try {
  JSON.parse(input);
} catch {
  // ^ Notice that our `catch` clause doesn't declare a variable.
  console.log("Invalid JSON given\n\n" + input);
}

checkJs/@ts-check 模式下的类型断言/强制类型转换语法

¥Type assertion/cast syntax in checkJs/@ts-check mode

TypeScript 2.5 引入了 在项目中使用纯 JavaScript 时，请声明表达式的类型 功能。语法是 /** @type {...} */ 注解注释，后跟一个带括号的表达式，该表达式的类型需要重新评估。例如：

¥TypeScript 2.5 introduces the ability to assert the type of expressions when using plain JavaScript in your projects. The syntax is an /** @type {...} */ annotation comment followed by a parenthesized expression whose type needs to be re-evaluated. For example:

ts
var x = /** @type {SomeType} */ AnyParenthesizedExpression;

去重和重定向的包

¥Deduplicated and redirected packages

在 TypeScript 2.5 中使用 Node 模块解析策略导入时，编译器现在将检查文件是否源自 “identical” 包。如果文件来自一个包，该包的 package.json 包含与之前遇到的包相同的 name 和 version 字段，则 TypeScript 会将自身重定向到最顶层的包。这有助于解决两个包可能包含相同的类声明，但包含导致它们在结构上不兼容的 private 成员的问题。

¥When importing using the Node module resolution strategy in TypeScript 2.5, the compiler will now check whether files originate from “identical” packages. If a file originates from a package with a package.json containing the same name and version fields as a previously encountered package, then TypeScript will redirect itself to the top-most package. This helps resolve problems where two packages might contain identical declarations of classes, but which contain private members that cause them to be structurally incompatible.

此外，通过避免从重复的包中加载 .d.ts 文件，这还可以减少编译器和语言服务的内存和运行时占用。

¥As a nice bonus, this can also reduce the memory and runtime footprint of the compiler and language service by avoiding loading .d.ts files from duplicate packages.

--preserveSymlinks 编译器标志

¥The --preserveSymlinks compiler flag

TypeScript 2.5 引入了 preserveSymlinks 标志，其行为与 Node.js 中的 --preserve-symlinks 标志 类似。此标志还表现出与 Webpack 的 resolve.symlinks 选项相反的行为（例如，将 TypeScript 的 preserveSymlinks 设置为 true 相当于将 Webpack 的 resolve.symlinks 设置为 false，反之亦然）。

¥TypeScript 2.5 brings the preserveSymlinks flag, which parallels the behavior of the --preserve-symlinks flag in Node.js. This flag also exhibits the opposite behavior to Webpack’s resolve.symlinks option (i.e. setting TypeScript’s preserveSymlinks to true parallels setting Webpack’s resolve.symlinks to false, and vice-versa).

在此模式下，对模块和包的引用（例如 import 和 /// <reference type="..." /> 指令）均相对于符号链接文件的位置进行解析，而不是相对于符号链接解析到的路径。为了举一个更具体的例子，我们将使用 Node.js 网站上的文档。

¥In this mode, references to modules and packages (e.g. imports and /// <reference type="..." /> directives) are all resolved relative to the location of the symbolic link file, rather than relative to the path that the symbolic link resolves to. For a more concrete example, we’ll defer to the documentation on the Node.js website.