JavaScript Changes in MongoDB 2.4

E4X Extensions

V8 does not support the non-standard E4X extensions. E4X provides a native XML object to the JavaScript language and adds the syntax for embedding literal XML documents in JavaScript code.

You need to use alternative XML processing if you used any of the following constructors/methods:

• XML()

• Namespace()

• QName()

• XMLList()

• isXMLName()

Destructuring Assignment

V8 does not support the non-standard destructuring assignments. Destructuring assignment "extract[s] data from arrays or objects using a syntax that mirrors the construction of array and object literals." - Mozilla docs

original = [4, 8, 15];
var [b, ,c] = a;  // <== destructuring assignment
print(b) // 4
print(c) // 15

Iterator(), StopIteration(), and Generators

V8 does not support Iterator(), StopIteration(), and generators.

InternalError()

V8 does not support InternalError(). Use Error() instead.

for each...in Construct

V8 does not support the use of for each...in construct. Use for (var x in y) construct instead.

var o = { name: 'MongoDB', version: 2.4 };
for each (var value in o) {
  print(value);
}

var o = { name: 'MongoDB', version: 2.4 };
for (var prop in o) {
  var value = o[prop];
  print(value);
}

Object.keys(o).forEach(function (key) {
  var value = o[key];
  print(value);
});

Array Comprehension

V8 does not support Array comprehensions.

Use other methods such as the Array instance methods map(), filter(), or forEach().

var a = { w: 1, x: 2, y: 3, z: 4 }
var arr = [i * i for each (i in a) if (i > 2)]
printjson(arr)

var a = { w: 1, x: 2, y: 3, z: 4 }
var arr = [];
Object.keys(a).forEach(function (key) {
  var val = a[key];
  if (val > 2) arr.push(val * val);
})
printjson(arr)

Multiple Catch Blocks

V8 does not support multiple catch blocks and will throw a SyntaxError.

try {
  something()
} catch (err if err instanceof SomeError) {
  print('some error')
} catch (err) {
  print('standard error')
}

Conditional Function Definition

V8 will produce different outcomes than SpiderMonkey with conditional function definitions.

function test () {
   if (false) {
      function go () {};
   }
   print(typeof go)
}

function test () {
  var go;
  if (false) {
    go = function () {}
  }
  print(typeof go)
}

function test () {
  'use strict';
  if (false) {
    function go () {}
  }
}

SyntaxError: In strict mode code, functions can only be declared at top level or immediately within another function.

String Generic Methods

V8 does not support String generics. String generics are a set of methods on the String class that mirror instance methods.

var name = 'MongoDB';
var lower = String.toLowerCase(name);

var name = 'MongoDB';
var lower = name.toLowerCase();
print(name + ' becomes ' + lower);

With V8, use the String instance methods instead of following generic methods:

Array Generic Methods

V8 does not support Array generic methods. Array generics are a set of methods on the Array class that mirror instance methods.

var arr = [4, 8, 15, 16, 23, 42];
function isEven (val) {
   return 0 === val % 2;
}
var allEven = Array.every(arr, isEven);
print(allEven);

var allEven = arr.every(isEven);
print(allEven);

With V8, use the Array instance methods instead of the following generic methods:

Array Instance Method toSource()

V8 does not support the Array instance method toSource(). Use the Array instance method toString() instead.

uneval()

V8 does not support the non-standard method uneval(). Use the standardized JSON.stringify() method instead.