Refactoring 2

Example code for weekly refactoring sessions.

Slides: https://bit.ly/39xupAQ

Notes

• Before doing the refactorings try to clear the decks and see how much you can improve the code only by renaming things.
• In the Introduce Parameter Object example, doesNotContain is a stylistic choice, feel free to extract contains as per the book.
  • Pick one style and stick with it, either verbose names or using !contains, remember when in rome.

Introduce Parameter Object

1. Create NumberRange class, move to number-range.ts:

  class NumberRange {
    constructor(
      readonly min: number,
      readonly max: number,
    ){}
  } 

2. Introduce range alongside existing parameters for readingsOutsideRange:

    const range = new NumberRange(operatingPlan.temperatureFloor, operatingPlan.temperatureCeiling)

    const alerts = readingsOutsideRange(station, operatingPlan.temperatureFloor, operatingPlan.temperatureCeiling, range)

3. Redirect usages of min and max to range.min and range.max in readingsOutsideRange:

    // from
    function readingsOutsideRange(station: Station, min: number, max: number, range: NumberRange): Reading[] {
        return station.readings.filter((r) => r.temp < min || r.temp > max)
    }
    
    // to
    function readingsOutsideRange(station: Station, min: number, max: number, range: NumberRange): Reading[] {
        return station.readings.filter((r) => r.temp < range.min || r.temp > range.max)
    }

4. Safe delete the unused min and max parameters:

    const alerts = readingsOutsideRange(station, range)

    function readingsOutsideRange(station: Station, range: NumberRange): Reading[] {
        return station.readings.filter((r) => r.temp < range.min || r.temp > range.max)
    }

5. Extract method doesNotContain on r.temp < range.min || r.temp > range.max in readingsOutsideRange:

    function doesNotContain(r: Reading, range: NumberRange): boolean {
      return r.temp < range.min || r.temp > range.max
    }

    export function readingsOutsideRange(station: Station, range: NumberRange): Reading[] {
      return station.readings.filter((r) => doesNotContain(r, range))
    }

6. Extract parameter on both occurrences of r.temp in doesNotContain:

    function doesNotContain(r: Reading, range: NumberRange, number: number): boolean {
      return number < range.min || number > range.max
    }

    export function readingsOutsideRange(station: Station, range: NumberRange): Reading[] {
      return station.readings.filter((r) => doesNotContain(r, range, r.temp))
    }

7. Safe delete the unused r parameter:

    function doesNotContain(range: NumberRange, number: number): boolean {
      return number < range.min || number > range.max
    }

    export function readingsOutsideRange(station: Station, range: NumberRange): Reading[] {
      return station.readings.filter((r) => doesNotContain(range, r.temp))
    }

8. Move doesNotContain to number-range.ts:

    export class NumberRange {
      constructor(readonly min, readonly max) {}
    }

    export function doesNotContain(range: NumberRange, number: number): boolean {
      return number < range.min || number > range.max
    }
    
    // In introduce-parameter-object.ts
    // import { doesNotContain } from './number-range'

9. Manually create method doesNotContain(number: number): boolean on NumberRange and delegate to doesNotContain:

    export class NumberRange {
      constructor(readonly min, readonly max) {}
      
      doesNotContain(number: number): boolean {
        return doesNotContain(this, number)
      }
    }

    export function doesNotContain(range: NumberRange, number: number) {
      return number < range.min || number > range.max
    }

10. Redirect call to doesNotContain in readingsOutsideRange to range.doesNotContain:

    function readingsOutsideRange(station: Station, range: NumberRange): Reading[] {
      return station.readings.filter((r) => range.doesNotContain(r.temp))
    }

11. Inline doesNotContain inside of number-range.ts:

    export class NumberRange {
      constructor(readonly min, readonly max) {}

      doesNotContain(number: number): boolean {
        return number < this.min || number > this.max
      }
    }

12. Remove unused import of doesNotContain in introduce-parameter-object.ts:

    import { NumberRange } from './number-range'