在将按键事件应用于 DOM 之前，如何确定按键事件之后的值是什么？

Question

I have a textbox which a user will be typing a number into. I want to limit that number to a max of 5 million (5000000). I want to prohibit any keypresses which would cause that number to go above 5 million. I know I could wait for keyup and use the value then, but I want to prevent the keypress entirely.

How can I determine what the value of the textbox would be after a given keydown/keypress event is applied BEFORE it actually happens?

Edit: To make it clear, consider this scenario:

The box currently contains the number "4000000".

The user highlights the last number and changes it to a 1, so that it becomes "4000001"

But the user COULD have simply added a 1 to the end. The keydown event would look exactly the same, but the result would be outside the range of values I want to allow.

I need to know what the value of the box would be if the keydown event were applied normally, but I need to know before it happens.

To be clear, this post is NOT about limiting a text field to numbers-only. I'm capable of doing that.

Answer 1

Is that what you looking for?

 var max = 4000000; var last_value; $('#someinput').on('input', function(e){ var value = $(this).val(); if(value<=max) last_value = value; else $(this).val(last_value); })

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <input id="someinput" type="number"/>