antonmedv/fx · error · Error

Cannot flatten

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.

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 delete key from
• Cannot get keys of
• Cannot get length of
• Cannot get unique values of
• Cannot get values of

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

Appendix: 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) {

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