If you have a named function, i.e., function funcName() {...} already defined, then the process is simply to identify onclick with the name of the function:
document.getElementById("myBtn").onclick = displayDate;function displayDate() { document.getElementById("demo").innerHTML=Date();}<button id="myBtn">Try it</button><p id="demo"></p>This can help keep your code better organized. A function can be quite long, and defining it singularly to a single onclick, and not some independent variable that can be referenced later, might end up creating spaghetti code.