antonmedv/fx · error · Error

Cannot list ${typeof x}

Error message

Cannot list ${typeof x}

What it means

list(x) prints each element of an array to the console and returns the internal skip symbol; for any non-array it throws 'Cannot list <type>'. It is a display helper, so it deliberately rejects objects/strings instead of iterating their properties.

Source

Thrown at internal/engine/stdlib.js:164

  throw new Error(`Cannot reverse ${typeof x}`)
}

function keys(x) {
  if (typeof x === 'object' && x !== null) return Object.keys(x)
  throw new Error(`Cannot get keys of ${typeof x}`)
}

function values(x) {
  if (typeof x === 'object' && x !== null) return Object.values(x)
  throw new Error(`Cannot get values of ${typeof x}`)
}

function list(x) {
  if (Array.isArray(x)) {
    for (const y of x) console.log(y)
    return skip
  }
  throw new Error(`Cannot list ${typeof x}`)
}

function del(key) {
  return function (x) {
    if (Array.isArray(x)) {
      const copy = [...x]
      copy.splice(key, 1)
      return copy
    }
    if (typeof x === 'object' && x !== null) {
      const copy = {...x}
      delete copy[key]
      return copy
    }
    throw new Error(`Cannot delete key from ${typeof x}`)
  }
}

View on GitHub (pinned to 4f31cd3a0c)

Solutions

1. Ensure the input is an array; wrap scalars: list([x])
2. For objects, list the entries: Object.entries(x) mapped to strings first
3. Default nullable values: list(x ?? [])
4. Guard with Array.isArray before calling list

Example fix

// before
list(result)
// after
list(Array.isArray(result) ? result : [result])

if (!Array.isArray(x)) throw new TypeError('list() requires an array; got ' + (x === null ? 'null' : typeof x))

function isListable(x) { return Array.isArray(x) }

try { list(x) } catch (e) { if (e.message.startsWith('Cannot list')) { console.log([x]) } else { throw e } }

When it happens

Trigger: list({a:1}), list('abc'), list(null), list(undefined), list(42) — any non-array passed where an array of items to print was expected.

Common situations: Logging results of a function that returned a single object instead of an array, listing entries of a config object (use Object.entries), or null from a failed fetch.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Cannot get length of ${typeof x}
• Cannot get unique values of ${typeof x}
• Cannot sort ${typeof x}
• Cannot flatten ${typeof x}
• Cannot reverse ${typeof x}

AI-assisted analysis of antonmedv/fx@4f31cd3a0c (2026-09-02). Data as JSON: /api/errors/ddecdacd99b77839. Report an issue: GitHub.