Prism · Composition

Calculated fields

Derived fields from raw input: the lines of a cart become a subtotal, tax and total. $with names the subtotal once, and $round fixes the digits.

The source

{
  items: [
    { name: 'Widget', price: 9.99, qty: 3 },
    { name: 'Gadget', price: 19.5, qty: 2 },
    { name: 'Sprocket', price: 4.25, qty: 5 },
  ],
  taxRate: 0.08,
}

The config

{
  $with: {
    let: {
      subtotal: {
        $sum: {
          over: {
            $map: {
              over: { $ref: '$.items' },
              as: 'item',
              body: {
                $mul: [
                  { $get: { from: { $var: 'item' }, path: ['price'] } },
                  { $get: { from: { $var: 'item' }, path: ['qty'] } },
                ],
              },
            },
          },
        },
      },
    },
    value: {
      subtotal: { $round: { value: { $var: 'subtotal' }, digits: 2 } },
      tax: {
        $round: {
          value: { $mul: [{ $var: 'subtotal' }, { $ref: '$.taxRate' }] },
          digits: 2,
        },
      },
      total: {
        $round: {
          value: {
            $add: [
              { $var: 'subtotal' },
              { $mul: [{ $var: 'subtotal' }, { $ref: '$.taxRate' }] },
            ],
          },
          digits: 2,
        },
      },
      itemCount: { $count: { over: { $ref: '$.items' } } },
    },
  },
}

What comes out

{
  subtotal: 90.22,
  tax: 7.22,
  total: 97.44,
  itemCount: 3,
}

Output matches the expected value

Runs in this page on the installed package. The input and the expected output are defined by the example.