Filter an array in Typescript

I had a typed array and wanted to remove elements with an empty value. In vanilla Javascript that's easy enough, but instead of regular strings I was using string unions to restrict the available options.

type Property = {
  measurements: Measurement[]
}
 
type Measurement = {
  type: MeasurementType
  value: number
  unit: MeasurementUnit
}
 
type MeasurementType =
  | "LOT_DEPTH"
  | "LOT_AREA"
  | "LIVING_AREA"
 
type MeasurementUnit =
  | "SQUARE_FEET"
  | "SQUARE_METERS"
  | "FEET"
  | "METERS"
 
function getProperty(input): Property {
  return {
    measurements: [
      {
        type: "LOT_DEPTH",
        value: input.lotDepth.value,
        unit: input.lotDepth.unit,
      },
      {
        type: "LOT_AREA",
        value: input.lotArea.value,
        unit: input.lotArea.unit,
      },
      {
        type: "LIVING_AREA",
        value: input.livingArea.value,
        unit: input.livingArea.unit,
      },
    ].filter((measurement) => measurement.value),
  }
}

Typescript was able to correctly infer the type of the measurements array, but as soon as I added the filter it broke. Why? Filter returns a regular array of regular objects, and the type information about the string unions was lost

Type 'string' is not assignable to type 'MeasurementName'

Solutions

The solution I ended up going with was to use type predicates to tell filter which type to use. I found out afterward that TS can infer correctly without the type predicate as long as it's split with an intermediate variable.

Type predicates

.filter((measurement): measurement is Measurement  => measurement.value)

Type predicates (JSDoc)

.filter(/** @type function(*): measurement is Measurement */ (measurement) => measurement.value)

Intermediate variable (JSDoc)

function getProperty(input): Property {
  /** @type {Measurement[]} */
  const measurements = [
    {
      type: "LOT_DEPTH",
      value: input.lotDepth.value,
      unit: input.lotDepth.unit,
    },
    {
      type: "LOT_AREA",
      value: input.lotArea.value,
      unit: input.lotArea.unit,
    },
    {
      type: "LIVING_AREA",
      value: input.livingArea.value,
      unit: input.livingArea.unit,
    },
  ]
 
  return {
    measurements: measurements.filter(
      (measurement) => measurement.value,
    ),
  }
}