Javascript code used to determine whether an element exists

1. Determine whether a form element exists (1)
Copy codeThe Code is as follows:
If ("periodPerMonth" in document. theForm)
{
Return true;
}
Else {
Return false;
}

2. Determine whether the page element exists
Copy codeThe Code is as follows:
If (document. getElementById ("XXX "))
{
// Exists
}

3. Determine whether a form element exists (2)
Copy codeThe Code is as follows:
If (document. theForm. periodPerMonth)
{
// Exists
}

Or
Copy codeThe Code is as follows:
If (typeof (document. theForm. periodPerMonth) = "object ")
{
// Exists
}

4. Determine whether a form exists
Copy codeThe Code is as follows:
If (document. theForm)
{
// Exists
}

5. Use Jquery to write scripts
Copy codeThe Code is as follows:
If ($ ("# someID"). length> 0 ){
$ ("# SomeID"). text ("hi ");
}