How to force array with one element to generate square brackets in json

To complement Santiago's helpful answer:

• The enumeration behavior of PowerShell's pipeline means that a receiving command fundamentally cannot tell the difference between pipeline input that was provided as (a) a single input object or (b) as a single-element array.

  • That is, the following two commands both send a single [int] instance through the pipeline:

    • (42 | ForEach-Object GetType).Name -> Int32
    • (@(42) | ForEach-Object GetType).Name -> Int32

• By contrast, when passing input as an argument, the target command can make such a distinction - if designed to do so - and ConvertTo-Json does.

  • However, it is rare for cmdlets to make this distinction - see GitHub issue #4242 for a discussion.

As an alternative to passing input by argument, PowerShell (Core) 7+ introduced the
-AsArray switch, which requests that even a single input object (which may have been a single-element array originally) be treated as an array in its JSON representation.

# PS v7+ only; ditto for @(42) as input.
42 | ConvertTo-Json -AsArray -Compress # -> '[42]'

As iRon points out, you can achieve the same outcome by ensuring that a given array - even if it contains just one element - is sent through the pipeline as a whole, which also works in Windows PowerShell.

• Note: While with ConvertTo-Json it's much simpler to pass an array as an argument to ConvertTo-Json, as shown in Santiago's answer, the techniques below may be of interest for commands that do not support passing array-valued arguments or support pipeline input only.

# Works in Windows PowerShell too.
# The unary form of the "," operator ensures that the array
# is sent *as a whole* through the pipeline.
, @(42) | ConvertTo-Json -Compress # -> '[42]'

The unary form of ,, the array constructor ("comma") operator constructs what acts as a transient, auxiliary array here:

• Its one and only element is the input array.
• When the pipeline enumerates this array, its one and only element - the array of interest - is sent as a whole through the pipeline.

There's a less obscure - but less efficient - alternative, using Write-Output with its -NoEnumerate switch:

# Works in Windows PowerShell too.
# -NoEnumerate prevents enumeration of the input array 
# and sends it through the pipeline as a whole.
Write-Output -NoEnumerate @(42) | ConvertTo-Json -Compress # -> '[42]'

Note:

• While the result is the same as with the v7+ -AsArray switch, the mechanism is different:

• With the auxiliary-array / non-enumeration technique, ConvertTo-Json truly receives an array as its one and only input object.

• With the v7+ -AsArray switch, when it receives a scalar (non-array) as its only input object, it still treats it as an array.

• If multiple input objects are received, -AsArray is a no-op, because even without this switch a JSON array must of necessity be output, given that ConvertTo-Json alway collects its input up front and then outputs a single JSON document for it.

• Do not use -AsArray in combination with an argument (as opposed to pipeline input), as that will result in a nested JSON array, at least as of this writing (PowerShell 7.3.4):

  ConvertTo-Json -AsArray -Compress @(42) # !! -> '[[42]]'   
  

  • GitHub issue #10952 discusses this behavior, which may change.

The design rationale behind PowerShell's enumeration behavior:

PowerShell is built around pipelines: data conduits through which objects stream, one object at a time.^[1]

PowerShell commands output to the pipeline by default, and any command can write any number of objects, including none - and that number isn't known in advance, because it can vary depending on arguments and external state.

• E.g., Get-ChildItem *.txt can situationally emit none, 1, or multiple objects.

Since the pipeline is just a stream of objects of unspecified count, there is no concept of an array in the pipeline itself, neither on input nor on output:

• On input, arrays (and most enumerables)^[2] are enumerated, i.e. the elements are sent one by one to the pipeline. Therefore, there is no difference between sending a scalar (single object) and sending a single-element array through the pipeline, as demonstrated above.

• On output, multiple objects are simply output one at a time (though it is possible, but rare, to send an array (or other list-like type) as a whole, but it is then itself just another, single output object in the pipeline).

  • It is only when you collect a pipeline's output that arrays come into play, of necessity:

    • A single output object needs no container, and can just be received as itself.

    • Multiple objects need a container, and PowerShell automatically creates a System.Object[] array to collect the output objects in.

^{[1] You can introduce buffering of multiple objects with the common -OutBuffer parameter, but the next command in a pipeline still receives the buffered objects one by one.}

^{[2] For details, see the bottom section of this answer.}