仅当对象存在时才从javascript中的html表单对象读取

Question

post submit I want to javascript/client side read user input

var h = document.getElementById("myTEXTbox").value;

but the textbox only exists depending earlier user choices. is the an if object exists in javascript?

Answer 1

check the return of getEelementById , if an element doesnt exist it will return null, and since it is a falsey value you can use it in a if statement.

var element = document.getElementById("myTEXTbox");
var h =  element ? element.value : "";

So if element is not null , h will be set to element.value otherwise it will be set to an empty string

Answer 2

If the object exists then it is an object, if not, it will return undefined!

So you can test it like this:

var element = document.getElementById("myTEXTbox"),
    h = (element)? element.value : '';

Basically, you are using this logic: If the element does exist " (element) " or alternatively (el!=undefined) then read the value, if not set the value of 'h' to '' (empty). This could be null or another value.

Answer 3

Try this:

var element = document.getElementById("myTEXTbox"),
    h = element ? element.value : null;