Not able to access schema properties in pre save middleware mongoose

Question

Getting one weird issue, I'm not able to access any of my schema properties inside pre save middleware. When I mouse over on that property I'm able to see this error :

"Property createdAt does not exist on type Document"

following is my code :

import * as mongoose from 'mongoose';
var StaffSchema = new mongoose.Schema({
    name : {
        firstName: {
            type: String,
            required: true
        },
        lastName: {
            type: String,
            required: true
        },
        middleName: {
            type: String,
            required: true
        },
        email: {
            type: String,
            required: true
        },
        signatureLine: {
            type: String,
            required: true
        },
        isActive: {
            type: Boolean,
            default: true
        },
        isWithoutSupervisorCoSign: {
            type: Boolean,
            default: false
        },
        isShowInSchedule: {
            type: Boolean,
            default: false
        }
    },
    createdAt: {
        type: Date
    },
    updatedAt: {
        type: Date
    }
});

StaffSchema.pre('save', function(next) {
// do stuff
this.createdAt = Date.now();
next();
});

export default mongoose.model('staff', StaffSchema);