Element: childElementCount property

The Element.childElementCount read-only property returns the number of child elements of this element.

Examples

let sidebar = document.getElementById("sidebar");
if (sidebar.childElementCount > 0) {
  // Do something
}

HTTP: Hypertext Transfer Protocol

HTTP guides

HTTP authentication

Browser detection using the user agent string (UA sniffing)

HTTP caching

HTTP Client hints

Compression in HTTP

Compression Dictionary Transport

HTTP conditional requests

Connection management in HTTP/1.x

Content negotiation

List of default Accept values

Using HTTP cookies

Cross-Origin Resource Sharing (CORS)

CORS errors

Reason: CORS header 'Access-Control-Allow-Origin' does not match 'xyz

Reason: CORS request did not succeed

Reason: CORS disabled

Reason: CORS request external redirect not allowed

Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers

Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Methods

Reason: Did not find method in CORS header 'Access-Control-Allow-Methods

Reason: expected 'true' in CORS header 'Access-Control-Allow-Credentials

Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel

Reason: CORS header 'Access-Control-Allow-Origin' missing

Reason: Multiple CORS header 'Access-Control-Allow-Origin' not allowed

Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*

Reason: CORS header 'Origin' cannot be added

Reason: CORS preflight channel did not succeed

Reason: CORS request not HTTP

Content Security Policy (CSP)

Content Security Policy errors and warnings

Content Security Policy: The page's settings blocked the loading of a resource: xyz

Evolution of HTTP

Fetch metadata

IFrame credentialless

HTTP messages

Media types (MIME types)

Common media types

Network Error Logging (NEL)

Overview of HTTP

Permissions Policy

Protocol upgrade mechanism

Proxy servers and tunneling

Proxy Auto-Configuration (PAC) file

HTTP range requests

Redirections in HTTP

A typical HTTP session

User-Agent reduction

HTTP reference

HTTP headers

Accept header

Accept-CH header

Accept-Encoding header

Accept-Language header

Accept-Patch header

Accept-Post header

Accept-Query header

Accept-Ranges header

Access-Control-Allow-Credentials header

Access-Control-Allow-Headers header

Access-Control-Allow-Methods header

Access-Control-Allow-Origin header

Access-Control-Expose-Headers header

Access-Control-Max-Age header

Access-Control-Request-Headers header

Access-Control-Request-Method header

Activate-Storage-Access header

Age header

Allow header

Alt-Svc header

Alt-Used header

Attribution-Reporting-Eligible header

Attribution-Reporting-Register-Source header

Attribution-Reporting-Register-Trigger header

Authorization header

Available-Dictionary header

Cache-Control header

Clear-Site-Data header

Connection header