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Principles · TypeScript

YAGNI: delete speculative options

Delete the settings nobody uses, so each new column is a two-line change.

15 steps5 chapters~5 min at 1.25×#yagni#dead code#over-engineering#configuration

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Java Kotlin C#C++Python PHP TypeScript+5 soon

The lesson plays right here. Pause anywhere, step back, or jump to a step below. Watch the TypeScript video on YouTube Code on GitHub

Watch the narrated video

Every step

Problem

Steps 1–4

Refactor

Steps 5–10

Payoff

Step 11

TypeScript bonus

Steps 12–13

Recap

Steps 14–15

The code: before and after

Before

/* config.json
{
  "export": {
    "format": "csv",
    "delimiter": ",",
    "header": true
  }
}
*/
type ExportConfig = {
  format: string;
  delimiter: string;
  header: boolean;
};

class OrderExport {
  private readonly format: string;
  private readonly delimiter: string;
  private readonly header: boolean;

  constructor(cfg: ExportConfig) {
    this.format = cfg.format;
    this.delimiter = cfg.delimiter;
    this.header = cfg.header;
  }

  export(orders: Order[]): string {
    let out = "";
    if (this.header && this.format === "csv") {
      out += `id${this.delimiter}total\n`;
    }
    for (const o of orders) {
      let row: string;
      switch (this.format) {
        case "csv":
          row = `${o.id}${this.delimiter}${o.total}`;
          break;
        case "json":
          row = `{"id":"${o.id}","total":${o.total}}`;
          break;
        case "xml":
          row = `<order id="${o.id}" total="${o.total}"/>`;
          break;
        default:
          throw new Error(`unknown format ${this.format}`);
      }
      out += row + "\n";
    }
    return out;
  }
}

class NightlyJob {
  constructor(
    private readonly config: ExportConfig,
    private readonly orders: OrderRepo,
    private readonly warehouse: Warehouse,
  ) {}

  run(): void {
    const today = this.orders.today();
    const csv = new OrderExport(this.config).export(today);
    this.warehouse.uploadCsv(csv);
  }
}

After

/* config.json
{}
*/

function exportOrders(orders: Order[]): string {
  let out = "id,total,postalCode\n";
  for (const o of orders) {
    const row = `${o.id},${o.total},${o.postalCode}`;
    out += row + "\n";
  }
  return out;
}

class NightlyJob {
  constructor(
    private readonly orders: OrderRepo,
    private readonly warehouse: Warehouse,
  ) {}

  run(): void {
    const today = this.orders.today();
    const csv = exportOrders(today);
    this.warehouse.uploadCsv(csv);
  }
}

Transcript, step by step

Problem

Step 1: An export with three settings Before

Every night, this job exports today's orders and uploads them to the warehouse. Order Export copies three settings from the config file at the top, a format, a delimiter and a header flag. It writes a header line, then one row per order in whichever format the config names. It can write CSV, JSON or XML, so it looks ready for whatever the business asks for next.

Step 2: One column, four edits Code smell

The warehouse now wants the postal code as a third column. So the header gets a new name, and every case in the format switch gets a new value. A speculative option is a setting built for a need nobody has yet, and this class copies three of them. You can see what they cost here in the header, here in the CSV case, here in JSON, and here in XML.

Step 3: Two formats nobody reads Code smell

Yet the config file only ever says CSV, and the only caller uploads CSV to the warehouse. So the JSON and XML cases never run, and nobody would notice if one of them broke. One new column costs four edits, and two of them land in formats nothing reads. A typo in the XML quotes would sit there unnoticed until someone finally turns XML on. And every option multiplies the paths a reader has to check before trusting a change.

Step 4: Back it out, then delete Plan

Let's back that change out. YAGNI, you aren't gonna need it, says you build a feature when someone needs it, and these two formats have nobody. Kent Beck put it as make the change easy, then make the easy change. So we delete the options nobody uses, one small step at a time, and the file the warehouse gets stays the same after each one.

Refactor

Step 5: Delete the XML case Remove Dead Code

No config value picks XML and no caller reads it, so the XML case goes first. The switch still has CSV, JSON and the default case, and the file the warehouse gets is the same as before.

Step 6: Delete the JSON case Remove Dead Code

JSON goes for the same reason, so the switch is down to the CSV case and a default. That default only throws for a format the config file never sets.

Step 7: Format can only be csv Remove Field

With one case left, format can only ever be CSV, so it stops being a field. The switch collapses into the CSV row, and the header test only checks the header flag. Format leaves the config file too, because nothing reads it anymore.

Step 8: The delimiter is always a comma Remove Field

The delimiter was a comma in the config and nowhere else, so we write the comma in place. The field goes, along with its line in the constructor, its member in the config type and its line in the config file.

Step 9: The header is always on Remove Field

Header is always true, so the path that skips the header line has never run. We delete the if, so the output starts with the header, and then the field goes because nothing reads it anymore. The config file is down to an export object with nothing in it.

Step 10: Nothing left to configure Remove Parameter

The constructor took a config and read nothing from it, so it goes, and the Export Config type goes with it. The nightly job builds Order Export with no arguments now, and it drops its config parameter. And the config file at the top is an empty object.

Payoff

Step 11: The postal code, in two edits Payoff

Now the postal code goes back in. It takes one new name in the header and one new value in the row, so that's two edits. The JSON and XML cases that took two of the four edits are gone, and there's one row line where there were three cases. YAGNI fit here because the config always said CSV and the only caller uploads CSV, so those options cost an edit on every change and gave nothing back. The append, the return and the nightly job didn't change.

TypeScript bonus

Step 12: A class with nothing to hold Code smell

With the settings gone, Order Export has no state left, and export never touches this. So the job builds an object only to call its one method and throw it away.

Step 13: A function is enough Plain function

In TypeScript a function can live at the top of the file, so the class becomes export orders. The body is the same five lines, and the job calls the function with today's orders. If the export ever needs state again, like a setting that really varies, the class can come back with it.

Recap

Step 14: Where we started, where we ended Before / after

On the left, three fields copied a format, a delimiter and a header flag out of the config. The format switch had three cases, and only the CSV one ever ran. On the right, there's one short function, and the config file is an empty object. It already writes the postal code, and it's still shorter than the switch was.

Step 15: When to reach for it Takeaways

Delete an option when the config only ever sets one value and no caller reaches the other branches. Keep it when a second user is real and dated, like a signed contract with a JSON partner, or when ops set the value per environment, because deleting it means writing it again against a deadline. You'll spot it by config values that never change, switch cases no caller reaches, and defaults that nobody overrides.