conditional readonly signature field

in my digital forms using pdf expert on ipad

i have ( submit by email ) button with mailDoc, mouse up run JavaScript action

i added another mouse up action

if(this.getField("CAP1Signature").value !=""){

this.getField("CAP1Signature").readonly = true;

}

////

if(this.getField("CAP1Signature").value =""){

this.getField("CAP1Signature").readonly = false;

}

to make the a signature field is readonly if the field is already signed

and (not) if the field is still blank

to ensure that the file will send with readonly signature

and also make a chance for the user if he forgot to sign before click on the button

my script is working half or the other half

i tried also

if(this.getField("CAP1Signature").value !=""){

this.getField("CAP1Signature").readonly = true;

}else{this.getField("CAP1Signature").readonly = false;}

i got the same result

any help please !?

thank you in advance