You will need to store the previous state so that you can send the opposite state.
Use context for this.
e.g.
[{"id":"83746a428adcbc17","type":"inject","z":"7eecf5f1d763605a","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":810,"y":432,"wires":[["056008d289d9ada1"]]},{"id":"056008d289d9ada1","type":"function","z":"7eecf5f1d763605a","name":"toggle on/off","func":"var state = context.get(\"state\") || false;\nstate = !state;\ncontext.set(\"state\", state);\nif (state) {\n msg.payload = {payload: \"ON\"}\n} else {\n msg.payload = { payload: \"OFF\" }\n}\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":990,"y":432,"wires":[["52182669c4a415b7"]]},{"id":"52182669c4a415b7","type":"debug","z":"7eecf5f1d763605a","name":"payload","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"payload","statusType":"auto","x":1180,"y":432,"wires":[]}]
