In Koa.js you don’t need to use some specific methods to make a JSON response.
For example, if you send a string:
app.use((ctx) => ctx.body = 'string');
the browser will show Content-Type equal to text/plain:
If you send an object or an array the browser will show Content-Type equal to application/json.
Examples:
app.use((ctx) => ctx.body = [1,2,3]);
app.use((ctx) => ctx.body = { a: 1, b: 2, c: [3, 4] });
Koa.js automatically converts ctx.body to JSON if it’s an object or an array.