Introduction to generating TypeScript definition files from JavaScript code. The demo code uses the compiler options "--allowJs" in combination with "--declaration" (TypeScript 3.7) to generate typings from JavaScript code.

1. TypeScript 3.7 Language
Compiler
Create declarations from JavaScript

2. Benefits
• JavaScript library authors can bootstrap definition files
• Automatically generated definitions can be compared with manually
typed definitions (using third-party tools like TSEvolve)
Limitations
• --module flag must be „amd“ or „system“ (doesn‘t work with „esnext“)
to use --out
• Exports get wrapped in modules
• Exported classnames can interfere with module name
2019/12/17

3. Demo
tsc src/Point.js --allowJs --declaration --out dist/Point.js --module system
• --allowJs combined with --declaration
(available with TypeScript 3.7)
2019/12/17

4. ./src/Point.js
export class Point {
constructor(x, y) {
this.x = x;
this.y = y;
}
equals(point) {
return this.x === point.x && this.y === point.y;
}
}
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5. ./dist/Point.d.ts
declare module "Point" {
export class Point {
constructor(x: any, y: any);
x: any;
y: any;
equals(point: any): boolean;
}
}
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6. Erroneous Output
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7. Fixed Definitions
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