ts-transformer-keys

A TypeScript custom transformer which enables to obtain keys of given type.

Requirement

TypeScript >= 2.4.1

How to use this package

This package exports 2 functions. One is keys which is used in TypeScript codes to obtain keys of given type, while the other is a TypeScript custom transformer which is used to compile the keys function correctly.

How to use keys

import { keys } from 'ts-transformer-keys';

interface Props {
  id: string;
  name: string;
  age: number;
}
const keysOfProps = keys<Props>();

console.log(keysOfProps); // ['id', 'name', 'age']

How to use the custom transformer

Unfortunately, TypeScript itself does not currently provide any easy way to use custom transformers (See https://github.com/Microsoft/TypeScript/issues/14419). The followings are the example usage of the custom transformer.

webpack (with ts-loader or awesome-typescript-loader)

See examples/webpack for detail.

// webpack.config.js
const keysTransformer = require('ts-transformer-keys/transformer').default;

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.ts$/,
        loader: 'ts-loader', // or 'awesome-typescript-loader'
        options: {
          // make sure not to set `transpileOnly: true` here, otherwise it will not work
          getCustomTransformers: program => ({
              before: [
                  keysTransformer(program)
              ]
          })
        }
      }
    ]
  }
};

Rollup (with rollup-plugin-typescript2)

See examples/rollup for detail.

// rollup.config.js
import resolve from 'rollup-plugin-node-resolve';
import typescript from 'rollup-plugin-typescript2';
import keysTransformer from 'ts-transformer-keys/transformer';

export default {
  // ...
  plugins: [
    resolve(),
    typescript({ transformers: [service => ({
      before: [ keysTransformer(service.getProgram()) ],
      after: []
    })] })
  ]
};

ttypescript

See examples/ttypescript for detail. See ttypescript's README for how to use this with module bundlers such as webpack or Rollup.

// tsconfig.json
{
  "compilerOptions": {
    // ...
    "plugins": [
      { "transform": "ts-transformer-keys/transformer" }
    ]
  },
  // ...
}

ts-jest

See examples/ts-jest for details. In order to use this transformer with ts-jest, you need to add a wrapper around it like this:

// ts-jest-keys-transformer.js
const keysTransformer = require('ts-transformer-keys/transformer').default;
const name = 'my-key-transformer';
const version = 1;
const factory = (cs) => (ctx) => keysTransformer(cs.program)(ctx);
// For ts-jest 26 use:
// const factory = (cs) => (ctx) => keysTransformer(cs.tsCompiler.program)(ctx);

module.exports = { name, version, factory };

And add it in jest.config.js like this:

  globals: {
    'ts-jest': {
      // relative path to the ts-jest-keys-transformer.js file
      astTransformers: { before: ['src/react/ts-jest-keys-transformer.js'] },
    },
  },

Note: ts-jest 26.4.2 does not work with this transformer (fixed in ts-jest 26.4.3). Also, for versions smaller than 26.2, you need to provide the transformer in an array instead, like this: astTransformers: { before: ['src/react/ts-jest-keys-transformer.js'] }

TypeScript API

See test for detail. You can try it with $ npm test.

const ts = require('typescript');
const keysTransformer = require('ts-transformer-keys/transformer').default;

const program = ts.createProgram([/* your files to compile */], {
  strict: true,
  noEmitOnError: true,
  target: ts.ScriptTarget.ES5
});

const transformers = {
  before: [keysTransformer(program)],
  after: []
};
const { emitSkipped, diagnostics } = program.emit(undefined, undefined, undefined, false, transformers);

if (emitSkipped) {
  throw new Error(diagnostics.map(diagnostic => diagnostic.messageText).join('\n'));
}

As a result, the TypeScript code shown here is compiled into the following JavaScript.

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
var ts_transformer_keys_1 = require("ts-transformer-keys");
var keysOfProps = ["id", "name", "age"];
console.log(keysOfProps); // ['id', 'name', 'age']

Note

• The keys function can only be used as a call expression. Writing something like keys.toString() results in a runtime error.
• keys does not work with a dynamic type parameter, i.e., keys<T>() in the following code is converted to an empty array([]).

class MyClass<T extends object> {
  keys() {
    return keys<T>();
  }
}

License

MIT