ASCII stands for American Standard Code for Information Interchange. It describes an encoding for letters, numbers and punctuation symbols that can be realized in seven bits. It uses only seven of the 8 bits for historical reasons to allow the eighth bit to be used for parity control when the characters are transmitted through serial interfaces.

Many of the character codes are reserved to send control signals to terminals and to manage the communications. Modern networking provides this capability outside of the character encoding.

There is an extended ASCII encoding that provides all 8 bits for character code mapping. This defines the upper 127 characters in addition to the lower 127 characters in the 7-bit representation.

There are many alternative interpretations of the ASCII character set that allow for national extensions to the character set. In some cases, this may only result in the replacement of a few currency symbols.

JavaScript uses the Unicode character set. The lower 127 characters of Unicode are purposely mapped to the ASCII character. ASCII is described here to provide help when you are exchanging data files with ASCII based systems or applications.

It may also be useful in some situations if you are using JavaScript to drive a serial interface to control some external system. Whether you could do that would depend on the hosting environment. A browser wouldn't give you those capabilities, but an embedded JavaScript interpreter in a process control system may well allow you to do that sort of thing.

This table summarizes the lower 127 characters in the ASCII character set.