sourcemap is not generated

i saw in webpack.config.js

if (isTest) {
    config.devtool = 'inline-source-map';
  }
  else if (isProd) {
    config.devtool = 'source-map';
  }
  else {
    config.devtool = 'eval-source-map';
  }

But when i run 'npm run build' , I cannot find sourcemap is generated. 

