Js connects with TypeScript

This time I will bring you the connection between Js and TypeScript, and what are the precautions for using Js and TypeScript. The following is a practical case, let’s take a look.

Preface

TypeScript is a superset of JavaScript types. This is a sentence introduced in the TypeScript documentation. So are they related?

My understanding is that TypeScript introduces the characteristics of a strongly typed language based on JavaScript. Developers use TypeScript syntax for programming development, and finally convert TypeScript into JavaScript through conversion tools.

Using TypeScript can avoid the pitfalls of weakly typed languages ​​caused by developing on native JavaScript. (What should I enter? What should I return after the call? Let’s take a look at the source code...)

Hmm! Very good, strongly typed JavaScript, very good. However, I can’t bear the meticulous humanistic care of many libraries in NPM o(TヘTo)

Don’t be afraid, many libraries now quietly support TypeScript. Even if they have no intention of supporting it, there are still big guys who make selfless contributions. Quietly help these libraries support TypeScript

This leads to the topic of this article, the TypeScript declaration file. I think it is a header file for the JavaScript library similar to the C language. Its existence is to help TypeScript introduce the JavaScript library.

What is a declaration file?

is very similar to C/C *.h header files: when you reference a third-party library (.lib/.dll/ .so/.a/.la), the C/C compiler cannot automatically recognize the exported names and function type signatures in the library, which requires the use of header files for interface declarations.

Similarly, the TypeScript declaration file is a TypeScript code file with the .d.ts suffix, but its role is to describe the type information of all exported interfaces within a JavaScript module (in a broad sense).

For the writing and specifications of TypeScript declaration files, please refer to the following official documents and excellent blog posts:

• ##https://www.tslang.cn/docs/handbook/ declaration-files/introduction.html

##http://www.jb51.net/article/138217.htm
According to the official document , there are the following two bundling methods:

Bundled with your npm package
Publish to @types organization## on npm

• #Bundling with the npm package is what I mentioned earlier. Developers can use it directly after npm installs a library in the ts project and imports it in the code file.

When some libraries are not officially maintained, you can use the second method. After npm installs a library, execute npm install @types/library name to install the declaration file of the library. The principle is that after TypeScript version 2.0, when you import a library and the specified library is not found in the configured include path, it will look for the library in @types of node_modules.

Generally speaking, the official recommendation is the first way to write the release statement document. Let me directly use an example to demonstrate the first bundling method.

Example

First initialize the TypeScript project, the directory structure is as follows:

tsconfig The .json configuration is as follows:

{
 "compilerOptions": {
 "target": "es5",
 /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
 "module": "commonjs",
 /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
 "allowJs": true,
 "outDir": "./dist",
 /* Redirect output structure to the directory. */
 /* Allow javascript files to be compiled. */
 "strict": true /* Enable all strict type-checking options. */
 },
 "include": [
 "src/**/*"
 ]
}

As you can see, I wrote a module a and bundled a declaration file with it. The content of module a, that is, src/a/index.js is as follows:

const NAME = 'A';
let call = (who) => {
 console.log('Hello ' + who + '! I am ' + NAME);
}
export default {
 call
}

The content of its declaration file is src/a/index.d.ts as follows:

declare namespace a {
 function call(who: string): void;
}
export default a;

At this time, we can introduce the a module in the entry file src/index.ts:

import a from './a';
a.call('Pwcong');

After executing tsc on the command line, js code can be generated in the directory dist:

Execute the command node ./dist/index.js to get the corresponding correct output.

We then simulate importing the released NPM library, create directory b under the node_modules directory, and initialize the Node project. At this time, the directory structure is as follows:

The content of node_modules/b/types/package.json is as follows:

{
 "name": "b",
 "version": "1.0.0",
 "main": "./src/index.js",
 "types": "./types/index.d.ts"
}

The content of node_modules/b/src/index.js is as follows:

const NAME = 'B';
let call = (who) => {
 console.log('Hello ' + who + '! I am ' + NAME);
}
module.exports = {
 call
}

声明文件 node_modules/b/types/index.d.ts 内容如下：

declare namespace b {
 function call(who: string): void;
}
export = b;

这时，我们修改 src/index.ts ：

import a from './a';
a.call('Pwcong');
import b = require('b');
b.call('Pwcong');

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