Case vs Lookup with type discrimination support

after.js

/**
 * Prop getter
 * @inspiredby https://blog.mariusschulz.com/2017/01/06/typescript-2-1-keyof-and-lookup-types
 */
function prop<T, K extends keyof T>(obj: T, key: K) {
  return obj.hasOwnProperty(key) ? obj[key] : undefined;
}

export function buildAddFieldParams(field: ModelFieldTypes, zone: 'DISCRETE' | 'CONTINUOUS' | 'AGGREGATE'): EncField {
  let lookup = {
    STRING: {
      AGGREGATE: {
        formula: `COUNT([${field.name}])`,
        variable: { type: 'CONTINUOUS' }
      },
      DISCRETE: {
        formula: `[${field.name}]`,
        variable: { type: 'DISCRETE' }
      }
    },
    DATE: {},
    DATETIME: {
      AGGREGATE: {
        formula: `COUNT([${field.name}])`,
        variable: { type: 'CONTINUOUS' }
      },
      CONTINUOUS: {
        formula: `DATETRUNC('day', [${field.name}])`,
        variable: { type: 'CONTINUOUS' }
      },
      DISCRETE: {
        formula: `DATEPART('year', [${field.name}])`,
        variable: { type: 'DISCRETE' }
      }
    }
  }
        
  return prop(prop(lookup, field.dataType), zone);
}

before.js

export function buildAddFieldParams(field: ModelFieldTypes, zone: 'DISCRETE' | 'CONTINUOUS' | 'AGGREGATE'): EncField {
  switch (field.dataType) {
    case 'BOOLEAN':
    case 'STRING':
      switch (zone) {
        case 'AGGREGATE':
          return {
            formula: `COUNT([${field.name}])`,
            variable: { type: 'CONTINUOUS' }
          };
        case 'CONTINUOUS':
        case 'DISCRETE':
          return {
            formula: `[${field.name}]`,
            variable: { type: 'DISCRETE' }
          };
      }
      break;
    case 'DATE':
    case 'DATETIME':
      switch (zone) {
        case 'AGGREGATE':
          return {
            formula: `COUNT([${field.name}])`,
            variable: { type: 'CONTINUOUS' }
          };
        case 'CONTINUOUS':
          return {
            formula: `DATETRUNC('day', [${field.name}])`,
            variable: { type: 'CONTINUOUS' }
          };
        case 'DISCRETE':
          return {
            formula: `DATEPART('year', [${field.name}])`,
            variable: { type: 'DISCRETE' }
          };
      }
  }
}