Declare JSON responses
An endpoint returns JSON and needs an explicit JSON media type.
Express.js · Module
res.type
Sets the response Content-Type from a MIME type or file extension. Values containing / are used literally; other values undergo extension-based MIME lookup.
How it works
res.type(type) sets the response’s Content-Type. A value containing / is used directly; otherwise Express treats it as a file extension and looks up the corresponding MIME type.
Syntax
res.type(type)Parameters
| Name | Type | Description |
|---|---|---|
type | String | A MIME type or a file extension used to determine the response Content-Type. |
Returns
Response
Key points
- The method sets the response’s Content-Type header based on the supplied value.
- The argument can be either a complete MIME type or a file extension.
- Values without
/are interpreted as extensions rather than literal media types. - In Express 4.x, extension resolution uses the mapping exposed through express.static.mime.lookup().
- An HTML extension resolves to the text/html media type.
- The json extension resolves to application/json.
Example: Set a JSON response type
const express = require('express');
const app = express();
app.get('/data', (req, res) => {
res.type('json').send({ ok: true });
});
app.listen(3000);Sets the response MIME type to JSON before sending the payload.
When to use it
Resolve a standard extension
A route serves content whose format is identified by a known extension.
Set a custom MIME type
An API must emit a vendor-specific or other custom media type.
Interview questions
QWhat does res.type() do?
It accepts a MIME type or an extension and returns the response object. Extensions are resolved through Express’s MIME mapping, while values containing / are applied literally.
Follow-up: What does res.type('json') do? It looks up the JSON MIME type and sets the Content-Type accordingly.
QHow are extensions handled?
res.type('.html') and res.type('html') both select the HTML MIME type. The leading dot is optional for extension lookup.
Follow-up: How would you set a custom media type? Pass the complete value, such as application/vnd.example+json, because it contains /.
QWhat does res.type() return?
The method changes the response Content-Type header and returns res, allowing response methods to be chained. It does not itself send the response.
Follow-up: Why might setting the type before send() matter? The later body-sending operation can use the selected type when constructing the HTTP response.
Related
res.set()res.get()res.send()express.static.mime.lookup()