Create Hashtable from JSON

JavaScriptSerializer is available since .NET3.5 (may be installed on XP, included in Win7 and newer), it's several times faster than Convert-FromJSON and it properly parses nested objects, arrays etc.

function Parse-JsonFile([string]$file) {
    $text = [IO.File]::ReadAllText($file)
    $parser = New-Object Web.Script.Serialization.JavaScriptSerializer
    $parser.MaxJsonLength = $text.length
    Write-Output -NoEnumerate $parser.DeserializeObject($text)
}

A little late to the discussion here, but in PowerShell 6 (Core) there is a -AsHashtable parameter in ConvertFrom-Json.

$json = @{Path="C:\temp"; Filter="*.js"} | ConvertTo-Json

$hashtable = @{}

(ConvertFrom-Json $json).psobject.properties | Foreach { $hashtable[$_.Name] = $_.Value }

Adapted from PSCustomObject to Hashtable