document.onpageshow is not a function错误求助：事件定义方式疑问

Hey there, let's break down exactly what's going on here and how to get your code working right.

First off: document.onpageshow isn't a function you call directly—that's probably where you're tripping up. It's an event handler property, meaning you assign a function to it (instead of invoking it like document.onpageshow()). That error almost always pops up when someone tries to run it as a function instead of setting it to handle the event.

Why This Happens #

• You’re calling it like a function: If your code has something like document.onpageshow(), that's the issue. This property holds a handler function, it isn't one itself.
• It’s been overwritten: Some other code in your project might have accidentally set document.onpageshow to a non-function value (like a string, number, or null), making it impossible to use as an event handler.

Correct Ways to Use the pageshow Event #

You've got two solid options to set up this event properly:

1. Assign a function directly to document.onpageshow

This is the simplest approach for single event handlers:

document.onpageshow = function(event) {
  // Check if the page was loaded from the browser's back/forward cache
  if (event.persisted) {
    console.log("Page loaded from browser cache");
  } else {
    console.log("Fresh page load");
  }
};

2. Use addEventListener() (Recommended)

This method is better if you need multiple listeners for the same event, or if you want to avoid overwriting existing handlers:

document.addEventListener('pageshow', function(event) {
  if (event.persisted) {
    console.log("Page restored from cache");
  } else {
    console.log("First-time page load");
  }
});

Debugging Overwritten Properties #

If you suspect document.onpageshow has been messed with by other code, check its type in your browser console:

console.log(typeof document.onpageshow);

If it returns anything other than 'function' or 'undefined', that means another script has changed it. Hunt down lines where document.onpageshow is assigned a non-function value (like document.onpageshow = "some text";) and fix those.

Just a quick note: The pageshow event fires whenever the page is displayed—including when it's pulled from the browser's back/forward cache (bfcache). It's super handy for resetting state that might have been left hanging when the user navigates back to the page.

内容的提问来源于stack exchange，提问作者alessandro308