ZWJ

ZWJ, short for Zero Width Joiner, is a non-printing formatting character in the Unicode standard. Its code point is U+200D, and it is categorized as a format character (Cf). The character has no visible glyph and occupies no space on screen, but it affects text rendering by signaling that adjacent characters should be joined into a single glyph or glyph sequence when shaping the text.

Use in complex scripts and emoji: In scripts with complex joining behavior, such as Arabic, Persian, or many Indic scripts, the ZWJ guides the shaping engine to connect letters across boundaries and to form ligatures where appropriate. It can also prevent or enforce joining in some contexts, depending on surrounding characters. It is distinct from the Zero Width Non-Joiner (U+200C), which explicitly prevents joining.

In emoji and symbol sequences, ZWJ is used to create compound emoji by linking multiple emoji characters into a single pictograph, such as multi-person sequences or family groups. The presence of a ZWJ within a sequence tells rendering engines to combine the components rather than place them separately.

Implementation and handling: ZWJ appears in input as an invisible character; it can be inserted with keystrokes, or via numeric references like ‍ or ‍. Developers should ensure Unicode-aware text processing to avoid misinterpretation; some fonts or environments may render differently or ignore it, which can affect search, alignment, or emoji rendering. Overall, ZWJ is a subtle but important tool in controlling text shaping and emoji composition in modern software.