Testing JSON matches TypeScript types

type.test.ts

import { writeFileSync, mkdirSync, existsSync } from 'fs'
import { resolve } from 'path'
import * as ts from 'typescript'

function checkForTypeErrors(jsonContent: string) {
  const tempDir = resolve('.tmp')
  const tempFilePath = resolve(tempDir, 'typecheck.ts')
  if (!existsSync(tempDir)) {
    mkdirSync(tempDir)
  }
  const tsContent = `
import { Data } from '../type.test'
const data = <Data>${jsonContent}`
  writeFileSync(tempFilePath, tsContent);
  const program = ts.createProgram([tempFilePath], {
    noEmit: true,
    strictNullChecks: true,
    lib: [ "es5", "es6" ],
    target: ts.ScriptTarget.ES2016
  });
  const result = program.emit();
  const allErrors = ts.getPreEmitDiagnostics(program).concat(result.diagnostics)
  return allErrors.filter(e => e.file && e.file.fileName === tempFilePath)
}

export interface Data {
  id: string
  count: number
}

it('passes when matches', () => {
  const errors = checkForTypeErrors(
    `{
      "id": "my id",
      "count": 123
    }`
  )
  expect(errors.length).toEqual(0)
})

it('highlights type mismatch', () => {
  const errors = checkForTypeErrors(
    `{
      "id": 123,
      "name": "test"
    }`
  )
  expect(errors.length).toEqual(1)
  expect(ts.flattenDiagnosticMessageText(errors[0].messageText, '\n')).toEqual(
`Type '{ \"id\": number; \"name\": string; }' cannot be converted to type 'Data'.
  Types of property 'id' are incompatible.
    Type 'number' is not comparable to type 'string'.`)
})