⟨WJ⟩
Zero width, zero breaks: the link that binds without taking up room.
The word joiner is the inverse of the zero width space: where that one authorizes the break, this one forbids it — adding no blank at all. An arrow or a symbol glued to its number cross the line together.
It inherited the trade from U+FEFF, which doubled as the BOM; since Unicode 3.2, binding without width is U+2060's job. The named entity is ⁠.
Loose: x → y (the arrow may be orphaned at the break)
With WJ: x⁠→⁠y (no width, no break)