Globals and Built-ins

These are the global APIs that Mikro.js adds on top of the standard QuickJS environment. For standard JavaScript built-ins (Array, Object, Promise, Map, Set, etc.), see the QuickJS-NG documentation.

Timers

function setTimeout(callback: (...args: any[]) => void, ms?: number): number
function clearTimeout(id: number): void

function setInterval(callback: (...args: any[]) => void, ms?: number): number
function clearInterval(id: number): void

Standard timer functions. These drive the Mikro.js event loop.

console

const console: {
  log(...args: unknown[]): void
  info(...args: unknown[]): void
  warn(...args: unknown[]): void
  error(...args: unknown[]): void
}

Logs to the serial console on device, or stdout in the simulator.

btoa / atob

function btoa(data: string): string
function atob(data: string): string

Base64 encoding and decoding. btoa encodes a binary (latin1) string to base64. atob decodes a base64 string back.

btoa('Hello') // "SGVsbG8="
atob('SGVsbG8=') // "Hello"

btoa throws a RangeError if the string contains characters outside the latin1 range. atob throws a SyntaxError on invalid base64 input.

TextEncoder / TextDecoder

class TextEncoder {
  encode(input?: string): Uint8Array
}

class TextDecoder {
  decode(input?: ArrayBufferView | ArrayBuffer): string
}

UTF-8 encoding and decoding.

AbortController / AbortSignal

class AbortController {
  readonly signal: AbortSignal
  abort(reason?: unknown): void
}

class AbortSignal {
  readonly aborted: boolean
  readonly reason: unknown
  throwIfAborted(): void
  onabort: (() => void) | null
  addEventListener(type: 'abort', fn: () => void): void
  removeEventListener(type: 'abort', fn: () => void): void
  static abort(reason?: unknown): AbortSignal
  static timeout(ms: number): AbortSignal
  static any(signals: AbortSignal[]): AbortSignal
}

Cooperative cancellation. Most useful with fetch for request timeouts:

import {fetch} from 'mikrojs/fetch'

// Cancel if no response within 5 seconds
const result = await fetch('https://api.example.com/data', {
  signal: AbortSignal.timeout(5000),
})

if (!result.ok && result.error.name === 'Aborted') {
  console.error('Request timed out')
}

Manual cancellation:

const controller = new AbortController()
setTimeout(() => controller.abort(), 3000)

const result = await fetch('https://slow.example.com', {
  signal: controller.signal,
})

On ESP32, aborting a fetch cancels the underlying HTTP task and frees its TLS buffers immediately, which can reclaim 16-40 KB of heap.

AbortError / TimeoutError

The default abort reason is an AbortError, and AbortSignal.timeout() uses a TimeoutError. Both are Error subclasses:

const signal = AbortSignal.abort()
signal.reason instanceof AbortError // true
signal.reason.name // "AbortError"

const timeout = AbortSignal.timeout(1000)
// after timeout fires:
timeout.reason instanceof TimeoutError // true
timeout.reason.name // "TimeoutError"

Difference from web standard

Browsers use DOMException with name: "AbortError" and name: "TimeoutError". Mikro.js uses dedicated AbortError and TimeoutError classes instead, because DOMException is a DOM API with a large spec surface that doesn't belong on a microcontroller. Code that checks error.name === "AbortError" works the same way in both environments.

AbortSignal.any() combines multiple signals:

const timeout = AbortSignal.timeout(10000)
const manual = new AbortController()

const result = await fetch(url, {
  signal: AbortSignal.any([timeout.signal, manual.signal]),
})

import.meta

Available inside ES modules:

Property	Type	Description
import.meta.url	string	URL of the current module
import.meta.main	boolean	Whether this is the entry module
import.meta.dirname	string	Directory of the current module
import.meta.basename	string	Filename of the current module
import.meta.path	string	Full path of the current module
import.meta.env	Record<string, string>	Environment variables (from NVS on device)

dirname, basename, path, and env are only available for file-based modules (not built-in mik: modules).

Why isn't fetch a global?

In browsers and Node.js, fetch is a global function. In Mikro.js, you import it from mikrojs/fetch instead. This is because Mikro.js's fetch returns a Result instead of throwing on network errors, so the signature is different from the standard fetch. Making it a global would be misleading since it doesn't behave the same way.

import {fetch} from 'mikrojs/fetch'

const result = await fetch('https://example.com')
if (!result.ok) {
  // network error, not an exception
}

WinterTC compatibility

Mikro.js is not WinterTC compatible, and full compliance is not a goal. Where feasible, we align with WinterTC APIs (e.g., fetch, TextEncoder, TextDecoder, AbortSignal, timers), but the runtime is designed for microcontrollers, not server-side JavaScript, and some APIs don't make sense in this context.