fix(compiler): return original file content and show warning on emit skip (#2410)

Closes #2407

Author: ahnpnl

src/compiler/ts-compiler.ts

@@ -262,7 +262,9 @@ export class TsCompiler implements TsCompilerInstance {
       this._doTypeChecking(fileName)
       /* istanbul ignore next (this should never happen but is kept for security) */
       if (output.emitSkipped) {
-        throw new Error(interpolate(Errors.CannotCompile, { file: fileName }))
+        this._logger.warn(interpolate(Errors.CannotProcessFile, { file: fileName }))
+
+        return fileContent
       }
       // Throw an error when requiring `.d.ts` files.
       if (!output.outputFiles.length) {

src/utils/messages.ts

@@ -17,7 +17,7 @@ export const enum Errors {
   GotUnknownFileTypeWithBabel = 'Got a unknown file type to compile (file: {{path}}). To fix this, in your Jest config change the `transform` key which value is `ts-jest` so that it does not match this kind of files anymore. If you still want Babel to process it, add another entry to the `transform` option with value `babel-jest` which key matches this type of files.',
   ConfigNoModuleInterop = 'If you have issues related to imports, you should consider setting `esModuleInterop` to `true` in your TypeScript configuration file (usually `tsconfig.json`). See https://blogs.msdn.microsoft.com/typescript/2018/01/31/announcing-typescript-2-7/#easier-ecmascript-module-interoperability for more information.',
   MismatchNodeTargetMapping = 'There is a mismatch between your NodeJs version {{nodeJsVer}} and your TypeScript target {{compilationTarget}}. This might lead to some unexpected errors when running tests with `ts-jest`. To fix this, you can check https://github.com/microsoft/TypeScript/wiki/Node-Target-Mapping',
-  CannotCompile = "Unable to process '{{file}}'. Please make sure that `outDir` in your tsconfig is neither `''` or `'.'`.",
+  CannotProcessFile = "Unable to process '{{file}}', falling back to original file content. Please make sure that `outDir` in your tsconfig is neither `''` or `'.'`. You can also configure Jest config option `transformIgnorePatterns` to ignore {{file}} from transformation",
 }
 
 /**