denoland/deno · error · TypeError

Failed to execute 'observe' on 'PerformanceObserver': 1 argu

Error message

Failed to execute 'observe' on 'PerformanceObserver': 1 argument required, but only 0 present.

What it means

PerformanceObserver.observe declares a default parameter (`options = { __proto__: null }`), so a missing argument or explicit undefined becomes an empty dictionary and instead trips the later "Either 'entryTypes' or 'type' must be specified" check (ext/web/15_performance.js:517-521). This specific TypeError — "1 argument required, but only 0 present" — only fires when null is passed explicitly, because null does not trigger the default parameter (ext/web/15_performance.js:499-507).

Source

Thrown at ext/web/15_performance.js:504

  constructor(callback) {
    const prefix = "Failed to construct 'PerformanceObserver'";
    webidl.requiredArguments(arguments.length, 1, prefix);
    if (typeof callback !== "function") {
      throw new TypeError(
        `${prefix}: The callback provided as parameter 1 is not a function.`,
      );
    }
    this[webidl.brand] = webidl.brand;
    this[_callback] = callback;
  }

  observe(options = { __proto__: null }) {
    webidl.assertBranded(this, PerformanceObserverPrototype);
    const prefix = "Failed to execute 'observe' on 'PerformanceObserver'";

    if (options === undefined || options === null) {
      throw new TypeError(
        `${prefix}: 1 argument required, but only 0 present.`,
      );
    }

    const { entryTypes, type } = options;

    if (entryTypes !== undefined && type !== undefined) {
      throw new TypeError(
        `${prefix}: Cannot specify both 'entryTypes' and 'type'.`,
      );
    }

    if (entryTypes === undefined && type === undefined) {
      throw new TypeError(
        `${prefix}: Either 'entryTypes' or 'type' must be specified.`,
      );
    }

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Always pass an options object: `observe({ type: 'measure', buffered: true })` or `observe({ entryTypes: ['mark', 'measure'] })`.
2. Default nullish configs with `?? {}` or a meaningful fallback: `observe(options ?? { entryTypes: ['measure'] })`.
3. Type the variable as an object, not `T | null`, before calling observe.

Example fix

// before
observer.observe(config.measureOptions ?? null);

// after
observer.observe(config.measureOptions ?? { entryTypes: ["measure"] });

const isObserveOptions = (v) => v != null && typeof v === "object";
observer.observe(isObserveOptions(options) ? options : { entryTypes: ["measure"] });

const isObserveOptions = (v) => v != null && typeof v === "object";

When it happens

Trigger: `observer.observe(null)`; `observer.observe(getOptions())` where the lookup returned null; spreading config that is typed as possibly null.

Common situations: Optional-config patterns defaulting to null; config loaders returning null for missing sections; code ported from browsers, where observe() with no args throws the same message and the mapping is surprising.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Failed to construct 'PerformanceObserver': The callback prov
• The property 'options.eval' must be false when 'filename' is
• Failed to execute 'observe' on 'PerformanceObserver': Cannot
• Failed to execute 'observe' on 'PerformanceObserver': Either
• Failed to execute 'observe' on 'PerformanceObserver': 'entry

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/435bcafeda8ee4c0. Report an issue: GitHub.