Inspection

You can customize the inspect behavior on Node.js and Deno.

// @see https://docs.deno.com/api/deno/~/Deno.inspect
const DENO_CUSTOM_INSPECT = Symbol.for("Deno.customInspect");

// @see https://nodejs.org/api/util.html
const NODEJS_UTIL_INSPECT_CUSTOM = Symbol.for("nodejs.util.inspect.custom");

// @see https://x.com/theMackabu/status/2044185796681637923
const ANT_SYMBOL_INSPECT = Symbol.inspect;

Usage

class A {
  public constructor(
    public x: number,
    public y: number,
  ) {}

  #inspect() {
    return `x: ${this.x}, y: ${this.y}`;
  }

  [DENO_CUSTOM_INSPECT]() {
    return this.#inspect();
  }

  [NODEJS_UTIL_INSPECT_CUSTOM]() {
    return this.#inspect();
  }

  [ANT_SYMBOL_INSPECT]() {
    return this.#inspect();
  }
}

Once done, you’ll see the output of console.log is modified!

console.log(new A(1, 2));
// x: 1, y: 2

Going deeper

We can actually go deeper by monkey patching existing classes to provide more debug!

for (const SYMBOL_INSPECT of [
  DENO_CUSTOM_INSPECT,
  NODEJS_UTIL_INSPECT_CUSTOM,
  ANT_SYMBOL_INSPECT,
]) {
  Blob.prototype[SYMBOL_INSPECT] = function () {
    return `Hello { world: ${this.size} }`;
  };
}

console.log(new Blob());
// Hello { world: 0 }