`ctx.response.length`

Gets or sets the `Content-Length` response header in bytes.

Syntax

ctx.response.length  // or ctx.length

Parameters

Name	Type	Required	Description
`value`	`number`	No	When used as a setter, explicitly sets the `Content-Length` header.

Returns

number | undefined — Content-Length in bytes, or `undefined` when not set and body length cannot be determined.

Examples

import Koa from 'koa';

const app = new Koa();

app.use(async (ctx) => {
  const data = JSON.stringify({ hello: 'world' });
  ctx.type = 'application/json';
  ctx.length = Buffer.byteLength(data);
  ctx.body = data;
});

app.listen(3000);

Output

Content-Length: 17
{"hello":"world"}

Notes

Koa automatically computes `Content-Length` for string and Buffer bodies. Manually setting it is useful when streaming responses where the total size is known in advance, enabling progress indicators in the client. For streams with unknown length, omit `Content-Length` and the response will use chunked transfer encoding.