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 ?