@gera2ld/ts-guard

Guard your objects in TypeScript to make sure all fields have the expected types.

Example:

interface ListResponse {
  error: number;
  list: Item[];
}

// BE somehow returns `{ error: 0 }` without `list`
const data: ListResponse = await fetchData(); // -> { error: 0 }

// TS guard makes sure it is as declared
const safeData: ListResponse = tsGuard(data); // -> { error: 0, list: [] }

Installation

$ pnpm install @gera2ld/ts-guard

Usage

Code Change

+ import { tsGuard } from '@gera2ld/ts-guard/shim';

  export function fetchData() {
-   const res = await axios.get<ListResponse>('/api/fetch-data');
+   // Guard the types of res.data
+   const res = tsGuard(await axios.get<ListResponse>('/api/fetch-data'), 'data');
    return res;
  }

Integration with Bundlers

Rollup

Add a plugin to rollup.conf.js:

import { tsGuardRollup } from '@gera2ld/ts-guard/rollup';

export default {
  // ...
  plugins: [
    tsGuardRollup({
      rootDir: 'src',
    }),
    // ...
  ],
};

Webpack

Approach 1: Using a plugin

Update your webpack.config.js:

import { TsGuardPlugin } from '@gera2ld/ts-guard/webpack';

export default {
  // ...
  plugins: [
    // ...
    new TsGuardPlugin({
      rootDir: 'src',
    }),
  ],
};

Note: it doesn't work with thread-loader.

Approach 2: Compiling ahead of time

First compile a subfolder with ts-guard in command-line:

$ npx @gera2ld/ts-guard <root_dir>

All .ts files within <root_dir> will be compiled to .js files in the same directory.

Make sure to tell Webpack to resolve to .js file if both .js and .ts exist. This is the default behavior so usually you don't need to do anything.

Then compile the project as usual.

Why

We cannot guarentee what we get from another service has the structure we expect even if when use TypeScript.

Take the example above, if BE returns { error: 0 } without list, our page might break:

<div>Total: {data.list.length}</div>

We don't want to check null for every field, which is why we use TypeScript. So we use ts-guard to ensure this.

Result

The compiled code looks like this:

import { $tsGuard$ } from './_ts_guard.js';

export function fetchData() {
  const res = $tsGuard$(
    /* ListResponse */ 1,
    await axios.get('/api/fetch-data'),
    'data',
  );
  return res;
}

Even if /api/fetch-data returns { error: 0 }, $tsGuard$ will patch the response into { error: 0, list: [] }.

Caveat

• Currently only arrays and objects are checked. Other types are ignored to minimize the overhead, but it is possible to support all types.
• If the absence of a field has a different meaning, you should either not pass it to tsGuard, or mark the field as optional (list?: Item[]).