Don't use a junction box to repair an extension cord.

No. That’s a bad idea and probably against electrical code. Extension cords are made for temporary use, not permanent splices inside junction boxes. Cutting out a damaged section and adding a box turns it into a fixed installation, which the cord wasn’t designed for, and it can create a fire or shock hazard.

Junction boxes are for in-wall or structural wiring where the cable is rated for that kind of connection. An extension cord’s flexible, stranded wires can loosen over time under tension or movement. The box will get tugged, the connections can fail, and you’re back to square one—with more risk.

Just replace the whole cord. Extension cords aren’t expensive. A repair like this might pass a quick test, but it’s not safe long-term.

Don’t risk a fire to save twenty bucks.