encodeURIComponent() escapes all characters except letters, digits, and -_.!~*'() using percent-encoding (%XX). Essential when building URLs with query parameters containing Cyrillic, spaces, or special characters. Unlike encodeURI(), it also encodes /, ?, #, and & — suitable for encoding parameter values, not full URLs.

Introduced in ES3 (1999) as a replacement for the deprecated escape(). Supported by all browsers without exception.

• Use for parameter values, not full URLs — for full URLs use encodeURI()
• spaces become %20, not + (unlike HTML form encoding)
• always encode user input before interpolating into a URL.