cleanup: pg utils (#3675)

* cleanup: `arrayString` code style

* cleanup: Move `Buffer.from` Node 4 compatibility code to common function

Reviewed-by: brianc

Author: charmander

packages/pg/lib/utils.js

@@ -11,35 +11,36 @@ function escapeElement(elementRepresentation) {
   return '"' + escaped + '"'
 }
 
+// Node.js v4 does not support those Buffer.from params
+const bufferFrom =
+  Buffer.from(new Uint8Array(1).buffer, 0, 0).length === 0
+    ? Buffer.from
+    : (arrayBuffer, byteOffset, length) => Buffer.from(arrayBuffer).slice(byteOffset, byteOffset + length)
+
 // convert a JS array to a postgres array literal
 // uses comma separator so won't work for types like box that use
 // a different array separator.
 function arrayString(val) {
   let result = '{'
   for (let i = 0; i < val.length; i++) {
     if (i > 0) {
-      result = result + ','
+      result += ','
     }
-    if (val[i] === null || typeof val[i] === 'undefined') {
-      result = result + 'NULL'
-    } else if (Array.isArray(val[i])) {
-      result = result + arrayString(val[i])
-    } else if (ArrayBuffer.isView(val[i])) {
-      let item = val[i]
+    let item = val[i]
+    if (item == null) {
+      result += 'NULL'
+    } else if (Array.isArray(item)) {
+      result += arrayString(item)
+    } else if (ArrayBuffer.isView(item)) {
       if (!(item instanceof Buffer)) {
-        const buf = Buffer.from(item.buffer, item.byteOffset, item.byteLength)
-        if (buf.length === item.byteLength) {
-          item = buf
-        } else {
-          item = buf.slice(item.byteOffset, item.byteOffset + item.byteLength)
-        }
+        item = bufferFrom(item.buffer, item.byteOffset, item.byteLength)
       }
       result += '\\\\x' + item.toString('hex')
     } else {
-      result += escapeElement(prepareValue(val[i]))
+      result += escapeElement(prepareValue(item))
     }
   }
-  result = result + '}'
+  result += '}'
   return result
 }
 
@@ -57,11 +58,7 @@ const prepareValue = function (val, seen) {
       return val
     }
     if (ArrayBuffer.isView(val)) {
-      const buf = Buffer.from(val.buffer, val.byteOffset, val.byteLength)
-      if (buf.length === val.byteLength) {
-        return buf
-      }
-      return buf.slice(val.byteOffset, val.byteOffset + val.byteLength) // Node.js v4 does not support those Buffer.from params
+      return bufferFrom(val.buffer, val.byteOffset, val.byteLength)
     }
     if (isDate(val)) {
       if (defaults.parseInputDatesAsUTC) {