Syntax

{$toInt: <arbitrary expression>}
{$toLong: <arbitrary expression>}
{$toDouble: <arbitrary expression>}
{$toDecimal: <arbitrary expression>}
{$toString: <arbitrary expression>}

Examples

> db.coll.insert([
  {_id: 0, x: "3.4"},
  {_id: 1, x: 0.99}
]);
> db.coll.aggregate([{
  $project: {
    intField: {$toInt: "$x"}
  }
}])
{_id: 0, intField: 3}
{_id: 1, intField: 0}

// Example 2
> db.coll.insert([
  {_id: 0, x: {a: "b"}},
  {_id: 1, x: 1.22},
  {_id: 2, x: "abc"}
]);
> db.coll.aggregate([{
  $project: {
    stringField: {$toString: "$x"}
  }
}])
{_id: 0, stringField: "{a: 'b'}"}
{_id: 1, stringField: "1.22"}
{_id: 2, stringField: "abc"}

Notes

• Each numeric conversion can convert from any numeric type or from a string (truncating if necessary).
• Calling $toInt on an int is a no-op, similarly for other conversions.

Errors

• If the input is of a type or value that cannot be converted:
  $toInt, $toLong, $toDouble, $toDecimal accept only int, long, double, decimal, and strings that can be interpreted as the above. $toString accepts anything that can be turned into output that is parseable by the mongo shell.

Old Description
There are situations where I want to use a number as (part of) a string but $concat will only take strings and not numbers. And there are other times where I have a string of a number "2012" and I want to use it as a number and there is no operator to do this.

Seems painful to fall back to map/reduce solely because someone saves a number or date as a string