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$toObject (expression operator)

Definition

$toObject

Converts a string to an object. If the value cannot be converted, $toObject errors. If the value is null or missing, $toObject returns null.

$toObject has the following syntax:

{
   $toObject: <expression>
}

$toObject takes any valid expression.

$toObject is a shorthand for the following $convert expression:

{ $convert: { input: <expression>, to: "object" } }

Behavior

Input Type Expectations

The following table describes the behavior of $toObject for different input types:

Input Type	Behavior
String	Returns a document corresponding to the content within the string. The string must contain characters that represent a valid JSON object.
Null or Missing	Returns null.

Parsing Rules

When converting a string to an object, $toObject:

Requires valid JSON syntax. Comments and trailing commas are not allowed.
Requires the top-level value to be an object. If the string does not represent an object, $toObject errors.
Does not interpret Extended JSON type wrappers such as $oid, $date, or Timestamp(...). These remain strings or nested objects in the result.
Preserves the last value when the object contains duplicate field names. Earlier values for the same field are discarded.

Numeric Type Mapping

$toObject converts numeric types based on their value and format:

Integers within the 32-bit signed range become int.
Integers outside the 32-bit range but within the 64-bit signed range become long.
Integers outside the 64-bit signed range become double, which can result in loss of precision.
Numbers with a decimal point or exponent notation become double.

Examples

The following table shows examples of using $toObject to convert strings to objects:

Example	Results
`$toObject: "{\"a\": 1, \"b\": 2}"`	{ a: 1, b: 2 }
`$toObject: "{}"`	{ }
`$toObject: "[]"`	Error: Input doesn't match expected type 'object'
`$toObject: "123"`	Error: Input doesn't represent valid JSON: Unexpected standalone value
`$toObject: "{\"nam\\u0000e\": \"foo\"}"`	Error: Input doesn't represent valid JSON: Illegal embedded null byte Note The string must contain characters that represent a valid object.
`$toObject: "{\"name\": \"fo\\u0000o\"}"`	{ name: 'fox00o' }
`$toObject: "{\"a\": 1, \"b\": 2, \"a\": 3}"`	{ a: 3, b: 2 } Note The last value for the same field is preserved.
`$toObject: "{\"foo\": null}"`	{ foo: null }
`$toObject: "{\"foo\": false}"`	{ foo: false }
`$toObject: "{\"__proto__\": {\"foo\": null}}"`	{ ['__proto__']: { foo: null } }
`$toObject: "{\"foo\": \"NaN\"}"`	{ foo: 'NaN' }
`$toObject: "{\"foo\": 123}"`	{ foo: 123 }
`$toObject: "{\"foo\": 4294967296}"`	{ foo: Long('4294967296') } Note The number is outside the 32-bit signed range, so it is converted to a long.
`$toObject: "{\"foo\": 1.123123}"`	{ foo: 1.123123 }
`$toObject: "{\"foo\": 1.2e+3}"`	{ foo: 1200 }
`$toObject: "{\"largePos\": 18446744073709551615}"`	{ largePos: 18446744073709552000 } Note The number is outside the 64-bit signed range, so it is converted to a double with precision loss.
`$toObject: "{\"largeNeg\": -18446744073709551615}"`	{ largeNeg: -18446744073709552000 } Note The number is outside the 64-bit signed range, so it is converted to a double with precision loss.
`$toObject: null`	null

Convert String to Object

Create a collection with strings stored in a field:

db.jsonStrings.insertOne({
  _id: 1,
  config: '{"feature": true, "threshold": 10}'
})

The following aggregation converts the string in config to an object:

db.jsonStrings.aggregate([
  {
    $project: {
      _id: 0,
      parsedConfig: { $toObject: "$config" }
    }
  }
])

This operation returns a document where parsedConfig is a nested document with a boolean and an integer value:

{ parsedConfig: { feature: true, threshold: 10 } }

Note

If the conversion operation encounters an error, the aggregation operation stops and throws an error. To override this behavior, use $convert instead.

Back

$toLong

$toObjectId