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问题:

The "onchange" event is triggered only when the USER enters some value. Why isn't possible to fire the event when I change the value automatically via Javascript ? Is there an alternative ?

Animation:

Code:

<!DOCTYPE html>

<html>
    <head>
        <script>
            document.addEventListener ("DOMContentLoaded", function () {
                var input = this.getElementsByTagName ("input")[0];
                var div = this.getElementsByTagName ("div")[0];
                var i = 0;
                var seconds = 5;

                div.innerHTML = "The following input should fire the event in " + seconds + " seconds";

                var interval = window.setInterval (function () {
                    i ++;

                    if (i === seconds) {
                        window.clearInterval (interval);

                        input.value = "Another example";

                        div.innerHTML = "Nothing ! Now try change the value manually";
                    }
                    else {
                        div.innerHTML = "The following input should fire the event in " + (seconds - i) + " seconds";
                    }
                }, 1000);

                input.addEventListener ("change", function () {
                    alert ("It works !");
                }, false);
            }, false);
        </script>

        <style>
            body {
                padding: 10px;
            }

            div {
                font-weight: bold;
                margin-bottom: 10px;
            }

            input {
                border: 1px solid black;
                border-radius: 3px;
                padding: 3px;
            }
        </style>

        <title>Event</title>
    </head>

    <body>
        <div></div>
        <input type = "text" value = "Example" />
    </body>
</html>

Thanks

回答1:

The vast majority of the time, you don't want an event to be fired when you change the value with code. In those cases where you do, you can fire a synthetic event on modern browsers via dispatchEvent. More here.

So in your specific example:

input.value = "Another example";
var event = document.createEvent("UIEvents"); // See update below
event.initUIEvent("change", true, true);      // See update below
input.dispatchEvent(event);

Live demo

Update: As Benjamin noted, since the above was written, initUIEvent has been replaced with the UIEvent constructor, so that would be:

input.value = "Another example";
var event = new UIEvent("change", {
    "view": window,
    "bubbles": true,
    "cancelable": true
});
input.dispatchEvent(event);

Live demo

Alternately, you can always just call whatever function you've bound to the change event directly, which is usually what I'd do. But sometimes you want to use actual events (for instance, when using the observer pattern) and ensure that anyone who is listening for the change is notified.

回答2:

Note that initUIEvent has been deprecated and removed from Web Standards as stated: developer.mozilla.org/en-US/docs/Web/API/UIEvent/initUIEvent

This is the same except that it doesn't use initUIEvent:

input.value = 'Another example';
var event = new UIEvent('change', {
    'view': window,
    'bubbles': true,
    'cancelable': true
});
input.dispatchEvent(event);

回答3:

If you are changing the value progammatically, you already know when this occurs, what's to say you can't call your own method, (the same perhaps that is called from the actual trigger event handler) when you change the value?

EDIT: otherwise, if you specifically need the actual Fire to occur, you can manually dispatch the event yourself too.

回答4:

The code of Crowder only gave me an TypeError (Not enough arguments to UIEvent.initUIEvent). Change it to this:

input.value = "Another example";
var event = document.createEvent("UIEvents");
event.initUIEvent("change", true, true, window, 1);
input.dispatchEvent(event);

and it works.