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ctx.response.body

Sets the response body; Koa infers Content-Type and Content-Length automatically based on the assigned value.

Since Koa 2 Spec ↗

Syntax

ctx.response.body = value  // or ctx.body = value

Parameters

NameTypeRequiredDescription
value string | Buffer | Stream | object | null Yes The response body. Accepted types: - `string` → `text/html` or `text/plain` - `Buffer` → `application/octet-stream` - `Stream` (Readable) → `application/octet-stream` - `object` / `Array` → `application/json` (JSON.stringify'd) - `null` → 204 No Content

Returns

void — Setter; returns nothing.

Examples

import Koa from 'koa';

const app = new Koa();

app.use(async (ctx) => {
  if (ctx.path === '/json') {
    ctx.body = { users: [{ id: 1, name: 'Alice' }] };
  } else if (ctx.path === '/text') {
    ctx.body = 'Plain text response';
  } else {
    ctx.body = null; // 204 No Content
  }
});

app.listen(3000);
Output 
GET /json → 200 {"users":[{"id":1,"name":"Alice"}]}
GET /text → 200 Plain text response
GET /     → 204 No Content

import { createReadStream } from 'node:fs';

app.use(async (ctx) => {
  ctx.type = 'application/pdf';
  ctx.body = createReadStream('./report.pdf');
});
Output 
GET /report → 200 (PDF binary stream)

Notes

Setting `ctx.body` to a non-null value automatically sets the status to `200` if it was previously `404`. The Content-Type can be overridden by setting `ctx.type` after (or before) setting `ctx.body`. For JSON, Koa uses `JSON.stringify` — install `koa-json` for pretty-printed output.

See also