[beta] warns the user if jest config transform matches js files without allowJs in tsconfig

src/ts-jest-transformer.spec.ts

@@ -1,5 +1,7 @@
 import stringify from 'fast-json-stable-stringify'
+import { ParsedCommandLine } from 'typescript'
 
+import { logTargetMock } from './__helpers__/mocks'
 import { TsJestTransformer } from './ts-jest-transformer'
 
 describe('configFor', () => {
@@ -31,8 +33,12 @@ describe('lastTransformerId', () => {
 describe('process', () => {
   let tr: TsJestTransformer
   let babel: any
+  let typescript: ParsedCommandLine
   let args: [string, string, any, any]
   const config = {
+    get typescript() {
+      return typescript
+    },
     shouldStringifyContent: jest.fn(),
     get babelJestTransformer() {
       return babel
@@ -53,12 +59,12 @@ describe('process', () => {
       .mockImplementation(() => config)
       .mockClear()
     config.shouldStringifyContent.mockImplementation(() => false).mockClear()
-    babel = { process: jest.fn(s => `babel:${s}`) }
+    babel = null
     config.tsCompiler.compile.mockImplementation(s => `ts:${s}`).mockClear()
+    typescript = { options: {} } as any
   })
 
   it('should process input without babel', () => {
-    babel = null
     expect(process()).toBe(`ts:${INPUT}`)
     expect(config.shouldStringifyContent.mock.calls).toMatchInlineSnapshot(`
 Array [
@@ -78,6 +84,7 @@ Array [
   })
 
   it('should process input with babel', () => {
+    babel = { process: jest.fn(s => `babel:${s}`) }
     expect(process()).toBe(`babel:ts:${INPUT}`)
     expect(config.shouldStringifyContent.mock.calls).toMatchInlineSnapshot(`
 Array [
@@ -113,7 +120,24 @@ Array [
     expect(process()).toMatchInlineSnapshot(`"ts:module.exports=\\"export default \\\\\\"foo\\\\\\"\\""`)
   })
 
+  it('should warn when trying to process js but allowJs is false', () => {
+    args[1] = '/foo/bar.js'
+    typescript.options.allowJs = false
+    const logs = logTargetMock()
+    logs.clear()
+    expect(process()).toBe(INPUT)
+    expect(logs.lines.warn).toMatchInlineSnapshot(`
+Array [
+  "[level:40] Got a \`.js\` file to compile while \`allowJs\` option is not set to \`true\` (file: /foo/bar.js). To fix this:
+  - if you want TypeScript to process JS files, set \`allowJs\` to \`true\` in your TypeScript config (usually tsconfig.json)
+  - if you do not want TypeScript to process your \`.js\` files, in your Jest config change the \`transform\` key which value is \`ts-jest\` so that it does not match \`.js\` files anymore
+",
+]
+`)
+  })
+
   it('should not pass the instrument option to babel-jest', () => {
+    babel = { process: jest.fn(s => `babel:${s}`) }
     args[3] = { instrument: true }
     expect(process()).toBe(`babel:ts:${INPUT}`)
     expect(config.babelJestTransformer.process.mock.calls).toMatchInlineSnapshot(`

src/ts-jest-transformer.ts

@@ -6,6 +6,7 @@ import { TsJestGlobalOptions } from './types'
 import { parse, stringify } from './util/json'
 import { JsonableValue } from './util/jsonable-value'
 import { rootLogger } from './util/logger'
+import { Errors, interpolate } from './util/messages'
 import { sha1 } from './util/sha1'
 
 /**
@@ -101,10 +102,18 @@ export class TsJestTransformer implements jest.Transformer {
       source = `module.exports=${JSON.stringify(source)}`
     }
 
-    // transpile TS code (source maps are included)
-    result = filePath.endsWith('.d.ts')
-      ? '' // do not try to compile declaration files
-      : configs.tsCompiler.compile(source, filePath)
+    // compilation
+    if (filePath.endsWith('.d.ts')) {
+      // do not try to compile declaration files
+      result = ''
+    } else if (!configs.typescript.options.allowJs && filePath.endsWith('.js')) {
+      // we've got a '.js' but the compiler option `allowJs` is not set or set to false
+      this.logger.warn({ fileName: filePath }, interpolate(Errors.GotJsFileButAllowJsFalse, { path: filePath }))
+      result = source
+    } else {
+      // transpile TS code (source maps are included)
+      result = configs.tsCompiler.compile(source, filePath)
+    }
 
     // calling babel-jest transformer
     if (babelJest) {

src/util/messages.ts

@@ -12,6 +12,7 @@ export enum Errors {
   NotMappingPathWithEmptyMap = 'Not mapping "{{path}}" because it has no target.',
   MappingOnlyFirstTargetOfPath = 'Mapping only to first target of "{{path}}" because it has more than one ({{count}}).',
   CannotPatchBabelCore6 = 'Error while trying to patch babel-core/lib/transformation/file: {{error}}',
+  GotJsFileButAllowJsFalse = 'Got a `.js` file to compile while `allowJs` option is not set to `true` (file: {{path}}). To fix this:\n  - if you want TypeScript to process JS files, set `allowJs` to `true` in your TypeScript config (usually tsconfig.json)\n  - if you do not want TypeScript to process your `.js` files, in your Jest config change the `transform` key which value is `ts-jest` so that it does not match `.js` files anymore',
 }
 
 export enum Helps {