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

Note:

• The answer below discusses providing input to ConvertTo-Json via the pipeline and the pitfall of using an empty or a single-element array as input, as well as workarounds.

  • As js2010's helpful answer shows, this pitfall can most easily be avoided if you pass such an array as an argument (e.g. ConvertTo-Json (@1)), also explained below.

  • For pipeline input, PowerShell (Core) 7+ now offers the convenient -AsArray switch as a way to solve the problem, discussed in the bottom section.

If $val is either the empty array, a scalar, or an array, use , @($val) | ConvertTo-Json 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.

  • GiHub issue #8393 asked for the treacherous default behavior to be changed; while that didn't happen, PowerShell 7+ now at least omits a warning then truncation occurs.

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 GitHub issue #10952.

^{[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.}