A substring is a portion of a string extracted from the original. It is a fundamental operation frequently used in text processing and data analysis.

JavaScript provides three methods: slice(), substring(), and substr(). slice() supports negative indices and is the most versatile. JavaScript string methods books explain the differences between these approaches.

When extracting substrings from Unicode text, be careful about surrogate pair and grapheme cluster boundaries. It is important to slice by character rather than by byte.

Python uses str[start:end] slice notation, while Java uses the substring() method. Text processing programming books compare substring operations across languages.