req.header vs req.headers in Express

The problem arises because in the HTTP protocol, headers are case-insensitive. This means that content-type, Content-Type, and coNTEnt-tYPe all refer to the same header, and the Express framework needs to be able to handle any of them.

The different between req.headers (the object) and req.header (the function) is simply this:

If you want to get a property from a Javascript object, the property name is case-sensitive. So req.headers['content-type'] will work; req.headers['Content-Type'] will not. Why does the lower case version work? Because the Express framework, in an attempt to handle all the different possible cases (remember, HTTP will allow anything), converts everything to lower case.

But the developers of Express recognize that you (the developer) might be looking for Content-Type and you might not remember to convert to lower case, so they provided a function, req.header, which will take care of that for you.

So, in short:

This is recommended:

const myHeader = req.header('Content-Type');

Use whatever case you want - the function will convert it to lower case and look up the value in req.headers.

This is not recommended:

const myHeader = req.headers['Content-Type'];

If you don't use a lower-case header name, you won't get what you expect.

The problem comes down to case-sensitivity.

When you look at the documentation for req.get (which is aliased by req.header), it states:

Returns the specified HTTP request header field (case-insensitive match). The Referrer and Referer fields are interchangeable.

The w3 standard indicates that headers should be case-insensitive:

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.

So it would appear that node http module, which express uses, just treats them all as lower-case to "save you steps" according to this github issue

You can see that the express framework req object actually utilizes the node module http:

var accepts = require('accepts');
var deprecate = require('depd')('express');
var isIP = require('net').isIP;
var typeis = require('type-is');
var http = require('http');
var fresh = require('fresh');
var parseRange = require('range-parser');
var parse = require('parseurl');

Furthermore, in the code you can see that the req.header method converts whatever you give it to lower-case:

req.get =
req.header = function header(name) {
  if (!name) {
    throw new TypeError('name argument is required to req.get');
  }

  if (typeof name !== 'string') {
    throw new TypeError('name must be a string to req.get');
  }

  var lc = name.toLowerCase();

  switch (lc) {
    case 'referer':
    case 'referrer':
      return this.headers.referrer
        || this.headers.referer;
    default:
      return this.headers[lc];
  }
};

Finally, the http module parses headers using the matchKnownFields function which automatically lower-cases any and all headers that aren't "traditional headers", in which case it is case-insensitive.

Here is the responsible snippet, that implements the behavior you are seeing:

if (lowercased) {
    return '\u0000' + field;
} else {
    return matchKnownFields(field.toLowerCase(), true);
}