Catch sparse arrays (#92)

* Test with sparse arrays

* Throw informative error when encoding a sparse array

lib/encoder.js

@@ -26,7 +26,7 @@ module.exports = function buildEncode (encodingTypes, options) {
       // and Buffer-like objects
       return bl([getBufferHeader(obj.length), obj])
     }
-    if (Array.isArray(obj)) return bl().append([ getHeader(obj.length, 0x90, 0xdc), ...obj.map(encode) ])
+    if (Array.isArray(obj)) return encodeArray(obj, encode)
     if (typeof obj === 'object') return encodeExt(obj, encodingTypes) || encodeObject(obj, options, encode)
     if (typeof obj === 'number') return encodeNumber(obj, options)
 
@@ -44,6 +44,22 @@ module.exports = function buildEncode (encodingTypes, options) {
 //
 //
 
+function encodeArray (array, encode) {
+  const acc = [ getHeader(array.length, 0x90, 0xdc) ]
+
+  // This has to be forEach; Array.prototype.map preserves missing values and
+  // Array.prototype.values yields them as undefined
+  array.forEach(item => {
+    acc.push(encode(item))
+  })
+
+  if (acc.length !== array.length + 1) {
+    throw new Error('Sparse arrays are not encodable in msgpack')
+  }
+
+  return bl(acc)
+}
+
 function encodeMap (map, options, encode) {
   const acc = [ getHeader(map.size, 0x80, 0xde) ]
   const keys = [ ...map.keys() ]

test/sparse-arrays.js

@@ -0,0 +1,18 @@
+'use strict'
+
+const test = require('tape').test
+const msgpack = require('../')
+
+test('throws when encoding sparse arrays', function (t) {
+  const encoder = msgpack()
+
+  t.deepEqual(encoder.decode(encoder.encode(new Array(0))), [])
+  t.throws(() => encoder.encode(new Array(1)), /Sparse arrays/)
+  t.throws(() => encoder.encode(new Array(100)), /Sparse arrays/)
+
+  const sparse = [1, 2, 3, 4]
+  delete sparse[3]
+  t.throws(() => encoder.encode(sparse), /Sparse arrays/)
+
+  t.end()
+})