How do you implement content negotiation so an endpoint returns JSON or CSV based on the Accept header in NestJS?

Q: How do you implement content negotiation so an endpoint returns JSON or CSV based on the Accept header in NestJS?

@Get('export') async export( @Headers('accept') accept: string, @Query() query: ExportQueryDto, @Res({ passthrough: true }) res: Response, ) { const data = await this.service.findAll(query); if (accept?.includes('text/csv')) { const csv = this.csvService.serialize(data); res.setHeader('Content-Type', 'text/csv'); res.setHeader('Content-Disposition', 'attachment; filename="export.csv"'); return new StreamableFile(Buffer.from(csv)); } if (accept?.includes('application/xml')) { res.setHeader('Content-Type', 'application/xml'); return this.xmlService.serialize(data); } // Default — JSON; NestJS handles Content-Type and serialization return data; } Check accept?.includes('text/csv') instead of strict equality — clients may send 'text/csv, */*'. Always provide a JSON fallback — it is the default and most widely supported format. Set Content-Type explicitly for non-JSON responses — NestJS defaults to application/json. For a reusable solution build a ContentNegotiationInterceptor that reads Accept and calls map(). Consider caching negotiated responses separately — CSV and JSON of the same data are different resources.

Difficulty: 5/10

Accept header, NestJS interceptors, Response serialization

Read the Accept header via @Headers('accept') and branch the response format. Use @Res({ passthrough: true }) to set Content-Type and return a StreamableFile for non-JSON formats while returning plain data for JSON. For a reusable solution build a content-negotiation interceptor that reads Accept and transforms via map() in the RxJS pipeline.

Content negotiation based on Accept header

Content negotiation implementation notes:

1
Check accept?.includes('text/csv') instead of strict equality — clients may send 'text/csv, /'.
2
Always provide a JSON fallback — it is the default and most widely supported format.

How do you implement content negotiation so an endpoint returns JSON or CSV based on the Accept header in NestJS?

Difficulty: 5/10

Accept header, NestJS interceptors, Response serialization

Read the Accept header via @Headers('accept') and branch the response format. Use @Res({ passthrough: true }) to set Content-Type and return a StreamableFile for non-JSON formats while returning plain data for JSON. For a reusable solution build a content-negotiation interceptor that reads Accept and transforms via map() in the RxJS pipeline.

Content negotiation based on Accept header

Content negotiation implementation notes:

1
Check accept?.includes('text/csv') instead of strict equality — clients may send 'text/csv, /'.
2
Always provide a JSON fallback — it is the default and most widely supported format.

How do you implement content negotiation so an endpoint returns JSON or CSV based on the Accept header in NestJS?

How do you implement content negotiation so an endpoint returns JSON or CSV based on the Accept header in NestJS?

Scenario Questions

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