antonmedv/fx · error · Error

Cannot flatten ${typeof x}

Error message

Cannot flatten ${typeof x}

What it means

flatten(x) calls Array.prototype.flat() (one level deep) and only accepts arrays; any other type throws 'Cannot flatten <type>'. Strings, objects, null and undefined are all rejected — use explicit conversion if you need those handled.

Source

Thrown at internal/engine/stdlib.js:141

    while (i < x.length) {
      res.push(x.slice(i, i += size))
    }
    return res
  }
}

function zip(...x) {
  const length = Math.min(...x.map(a => a.length))
  const res = []
  for (let i = 0; i < length; i++) {
    res.push(x.map(a => a[i]))
  }
  return res
}

function flatten(x) {
  if (Array.isArray(x)) return x.flat()
  throw new Error(`Cannot flatten ${typeof x}`)
}

function reverse(x) {
  if (Array.isArray(x)) return x.reverse()
  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) {

View on GitHub (pinned to 4f31cd3a0c)

Solutions

1. Confirm the input is an array with Array.isArray before flattening
2. If you have an object of arrays, flatten its values: Object.values(x).flat()
3. Default nullable inputs: flatten(x ?? [])
4. For deeper nesting combine with map: x.map(v => Array.isArray(v) ? v : [v]).flat()

Example fix

// before
const all = flatten(grouped)
// after
const all = flatten(Object.values(grouped))

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

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

let flat; try { flat = flatten(x) } catch (e) { if (e.message.startsWith('Cannot flatten')) { flat = [] } else { throw e } }

When it happens

Trigger: flatten({a:[1]}), flatten('abc'), flatten(null), flatten(undefined), or flatten on a value an upstream step returned as an object/scalar instead of an array.

Common situations: Grouping results (groupBy output is an object, not an array) then flattening, JSON APIs returning an object wrapper instead of an array, or null from a missing config key.

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 reverse ${typeof x}
• Cannot get keys of ${typeof x}

AI-assisted analysis of antonmedv/fx@4f31cd3a0c (2026-09-02). Data as JSON: /api/errors/156cd33295b9e3a6. Report an issue: GitHub.