In this section, you can find basic examples of how to work with the SharePoint fields using JavaScript.
For more examples, check out Working with form fields in JavaScript article. If you are not familiar with the JavaScript framework, get started with the JavaScript basics.
Note
The field is only accessible once the form is rendered, so all calls to the field must be inside fd.spRendered event:
fd.spRendered(function(){
//make field required
fd.field('Field1').required = true;
});