Typescript type checking test assertions

1-typecheck.js

const ts = require('typescript')

const DefaultOptions = {
  noEmit: true,
  target: ts.ScriptTarget.ES2019,
  module: ts.ModuleKind.CommonJS
}

function typecheck(filepath, options = DefaultOptions) {
  const program = ts.createProgram([filepath], options)
  const emitResult = program.emit()
  const diagnostics = ts.getPreEmitDiagnostics(program).concat(emitResult.diagnostics)
  return formatDiagnostics(diagnostics)
}

function formatDiagnostics(diagnostics) {
  return diagnostics.map((diagnostic) => {
    const { messageText: info, file } = diagnostic
    const result = {
      message: info.messageText,
      category: info.category,
      code: info.code
    }
    if (file) {
      const { line, character } = file.getLineAndCharacterOfPosition(diagnostic.start)
      result.file = file.fileName
      result.line = line + 1
      result.character = character + 1
    }
    return result
  })
}

module.exports = { typecheck }

2-input.ts

interface Foo {
  bar: string
  baz?: number
}

const foo: Foo = {
  bar: 1,
  baz: 'baz',
  qux: null
}

3-test.js

const { strictEqual } = require('assert')
const { describe, it } = require('mocha')
const { typecheck } = require('./typecheck')

describe('Type checking assertions', () => {
  it('Should fail with an invalid file', () => {
    const errors = typecheck('./input.ts')
    strictEqual(errors.length, 3)
  })
})