expressjs/express · error · TypeError

Content-Type cannot be set to an Array

Error message

Content-Type cannot be set to an Array

What it means

Thrown by `res.set`/`res.header` when the field is `Content-Type` and the value is an array. Content-Type is a singleton header — a response can only have one — and Express also passes the value through `mime.contentType()` to append a charset, which requires a single string.

Source

Thrown at lib/response.js:677

 * the charset if not present using `mime.contentType()`.
 *
 * @param {String|Object} field
 * @param {String|Array} val
 * @return {ServerResponse} for chaining
 * @public
 */

res.set =
res.header = function header(field, val) {
  if (arguments.length === 2) {
    var value = Array.isArray(val)
      ? val.map(String)
      : String(val);

    // add charset to content-type
    if (field.toLowerCase() === 'content-type') {
      if (Array.isArray(value)) {
        throw new TypeError('Content-Type cannot be set to an Array');
      }
      value = mime.contentType(value)
    }

    this.setHeader(field, value);
  } else {
    for (var key in field) {
      this.set(key, field[key]);
    }
  }
  return this;
};

/**
 * Get value for header `field`.
 *
 * @param {String} field
 * @return {String}

View on GitHub ↗ (pinned to a3714473fe)

Solutions

1. Set a single string: `res.set('Content-Type', 'application/json')` or use `res.type('json')`.
2. Never use res.append for Content-Type — use res.set, which replaces the value.
3. When copying headers from an upstream response, join or pick a single value for Content-Type before setting it.

Example fix

// before
res.append('Content-Type', 'application/json'); // throws if already set
// after
res.set('Content-Type', 'application/json');

if (Array.isArray(type)) {
  throw new TypeError('Content-Type must be a single string, got an array');
}
res.set('Content-Type', type);

function isSingleContentType(v) {
  return typeof v === 'string' && !Array.isArray(v);
}

When it happens

Trigger: `res.set('Content-Type', ['text/html', 'application/json'])`, `res.append('Content-Type', ...)` after a previous value (append concatenates prior and new values into an array and calls res.set), or `res.type()`/`res.set` fed an array-valued variable.

Common situations: Using res.append for Content-Type when a value already exists; copying headers from another response object where values are arrays; forwarding proxied headers wholesale (`res.set(upstream.headers)`) when a header value is an array.

Related errors

• path must be a string to res.sendFile
• Url must be a string
• name argument is required to req.get
• name must be a string to req.get
• app.use() requires a middleware function

AI-assisted analysis of expressjs/express@a3714473fe (2026-07-31). Data as JSON: /data/errors/42edae4ec9cbf5b6.json. Report an issue: GitHub ↗.