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//Assign a system calculated date based on data entry in another component.
var current = new Date(data.dateSigned);
if(data.signature && current.toString() === 'Invalid Date')
{
    var d = new Date();
    value = d.toLocaleDateString();
}
else
    value = data.dateSigned;

Add state control to check boxes to mimic radio button behavior

This script uses a Text Component (checkState) to preserve the last selected Checkbox Component.  Use two or more checkboxes (this example shows 3) and adjust the script accordingly.  In general, you should use a Radio Component to achieve this functionality.  This is an advanced script and it's very easy to miss updating a checkbox name, which can cause unexpected behavior.  Be sure to carefully inspect your script for errors when deploying.  A working form example is available here.

//Add state control to checkboxes to mimic radio button behavior.
if(data.check1 === true  && (data.checkState == '2' || data.checkState == '3'))
    {
        value = '1';
        data.check2 = false;
        data.check3 = false;
    }
else if (data.check2 === true && (data.checkState == '1' || data.checkState == '3'))
{
    value = '2';
    data.check1 = false;
    data.check3 = false;
}
else if (data.check3 === true && (data.checkState == '1' || data.checkState == '2'))
{
    value = '3';
    data.check1 = false;
    data.check2 = false;
}
else if (data.check1 === true)
    value = '1';
else if (data.check2 === true)
    value = '2';
else if (data.check3 === true)
    value = '3';
else
    value = '';

Populate Username or Email Address of Logged in User

You can populate the Name or Email Address of a logged in user by placing the below code in the Calculated Value Rule area on the Data Tab.

//Capture the email of the current logged in user into the value of the selected control
value = Formio.getUser().data.email


//Capture the name of the current logged in user into the value of the selected control
value = Formio.getUser().data.name

You can couple this rule with an if statement in a workflow to ensure it only fires once and does not overwrite the name/email if the form is loaded in additional workflow steps.

Populate Today's Date in a Text Field

You can populate today's date in a field for the purposes of tracking when a form was initally submitted and more.  Its recommended you use this rule as a Custom Default value if you only want it to fire once.

var d = new Date();
value = d.toLocaleDateString();

You can modify the date format in any way you want, for example:

var d = new Date();
var options = {
	year: "2-digit",
	month: "2-digit",
	day: "2-digit"
};
value = d.toLocaleDateString('en', options);