How to convert powershell array to json using ConvertTo-Json?

Question

Observe:

C:\> [array]@(1,2) | ConvertTo-Json
[
    1,
    2
]
C:\> [array]@(1) | ConvertTo-Json
1
C:\> [array]@() | ConvertTo-Json
C:\>

(I expect [1] and [] from the last two cases respectively)

So, if I want to use the standard ConvertTo-Json method, how do I do it reliably, even when the array contains 1 or 0 elements?

Note, that post-processing the result is not feasible when the array is part of a complex object converted to json.

EDIT 1

C:\> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.17763.592
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.17763.592
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1


C:\> [array]@(1) | ConvertTo-Json -AsArray
ConvertTo-Json : A parameter cannot be found that matches parameter name 'AsArray'.
At line:1 char:30
+ [array]@(1) | ConvertTo-Json -AsArray
+                              ~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [ConvertTo-Json], ParameterBindingException
    + FullyQualifiedErrorId : NamedParameterNotFound,Microsoft.PowerShell.Commands.ConvertToJsonCommand

C:\>

Answer 1

^{PowerShell Core (v6+) offers the convenient -AsArray switch, discussed in the bottom section.}

---

If $val is either the empty array, a scalar, or an array, send , @($val) to ConvertTo-Json via the pipeline to ensure that it gets serialized as an array:

if (-not $IsCoreCLR) {  # Workaround for Windows PowerShell
 # Only needed once per session.
 Remove-TypeData -ErrorAction Ignore System.Array
}

# Send an empty array, a single object, and an array...
@(), 1, (1, 2) | ForEach-Object { 
  # ... and ensure that each input ($_) serializes as a JSON *array*.
  , @($_) | ConvertTo-Json 
}

Note:

• The need for the Windows PowerShell workaround is explained in this answer.

• ,, the array-construction operator, is used here in its unary form to provide an auxiliary, single-element wrapper array in order to send the array as a whole (as a single object) through the pipeline; by default, sending an array (enumerable) to the pipeline sends its elements one by one; note that this is fundamental pipeline behavior, irrespective of the cmdlets involved.

• @(...), the "array-guarantor" operator (array-subexpression operator), ensures that $_ is an array, that is, it wraps the operand in an array, unless it already is one (loosely speaking^[1]); this is necessary to cover the case of $_ containing only a single object (scalar; 1 in this case).

• A general caveat: ConvertTo-Json quietly limits its serialization depth to 2 by default, which results in quiet data loss with more deeply nested input; use the -Depth parameter as needed.

  • This SO post discusses the problem.

  • This related GiHub issue asks for the treacherous default behavior to be changed - make your voice heard there, if you agree.

The above yields the following - note how each input was serialized as an array:

[]
[
  1
]
[
  1,
  2
]

Alternatively, you can pass the inputs as arguments to ConvertTo-Json with @($val):

# Same output as above.
@(), 1, (1,2) | ForEach-Object { ConvertTo-Json @($_) }

A positional argument implicitly binds to the -InputObject parameter, which does not enumerate its argument and therefore binds arrays as-is. Therefore you only need the "array guarantor" @() in this case (not also a wrapper array with , ).

---

PowerShell Core now offers an -AsArray switch, which directly ensures that the input is serialized as an array, even if there's only a single input object:

PS> 1 | ConvertTo-Json -AsArray
[
  1
]

However, given that empty arrays result in no data being sent through the pipeline, you still need a wrapper array if the input is the empty array and you then mustn't use -AsArray:

# Note:
#   @() | ConvertTo-Json -AsArray
# would result in NO output.
# Use `, ` to wrap the empty array to ensure it gets sent through
# the pipeline and do NOT use -AsArray
PS> , @() | ConvertTo-Json -Compress

[]

Alternatively, again pass the empty array as an argument:

PS> ConvertTo-Json @() -Compress # Do NOT use -AsArray

[]

The problem is that -AsArray unconditionally wraps its input in a JSON array, so that something that already is an array is wrapped again:

PS> ConvertTo-Json -AsArray @() -Compress

[[]]  # *nested* empty arrays

That -AsArray does not act as an array "guarantor" the way that @(...) does is discussed in this GitHub issue.

---

^{[1] If the operand is a scalar (single object), it is wrapped in a single-element [object[]]; if the operand already is an array or is an enumerable, the elements are enumerated and captured in a new [object[]] array.}