How to get a value of some other field in the same form ?

[vivekiyer114]

For example : -

'title':[{
           rule:'required',
           message:'Required'
       },{
           rule:(val,params,context) => {
               console.log(this.a)
               if (val.indexOf('/') !== (-1)){
                   throw new Error('Title cannot contain slashes')
               }
           },
           message:'Title cannot contain slashes'
       }],
       'description':[{
           rule:'required',
           message:'Required'
       },{
           rule:'minLength',
           params:'20',
           message:'Description should be atleast 20 characters'
       }],
       'categories':[{
           rule:'minLength',
           params:'1',
           message:'Required'
       }],

In the title field validation rule ,
how to get the value of categories ?