I use dotnet-watch a bunch and have used a version via grunt/gulp a little in the past as well.

In front of me, I had some TypeScript, tsc and an app.ts file. I wanted to watch it and trigger tsc automatically. But there was no csproj, no NuGet, and no npm in sight.

I went searching far and wide for solutions, and some clever people have some insight to add to the quest:

From these, I cobbled together what is now in my Microsoft.PowerShell_profile.ps1 file:

# Watch in Powershell
function Invoke-Watch {
    param(
        [string]$Folder,
        [string]$Filter = "*.*",
        [scriptblock]$Action,
        [Switch]$IncludeSubdirectories = $true
    )
    $watcher = New-Object IO.FileSystemWatcher $folder, $filter -Property @{ 
        IncludeSubdirectories = $IncludeSubdirectories
        EnableRaisingEvents = $true
    }

    Write-Host "Now Watching (ctrl+c to stop)"
    $global:PSWatchFileChanged = $false
    $onChange = Register-ObjectEvent $Watcher Changed -Action {$global:PSWatchFileChanged = $true}
    try{
        
        while($true) {
            & $Action
            while($global:PSWatchFileChanged -eq $false) {
                Start-Sleep -Milliseconds 100
            }
            $global:PSWatchFileChanged = $false
        }
    }
    finally{
        Unregister-Event -SubscriptionId $onChange.Id
    }
}
New-Alias -Name watch -Value Invoke-Watch

To use it, You pass a few parameters to the script:

# Watch tsc compile my ts files
watch -Folder "${PWD}" =Filter "*.ts" -Action { tsc }

At some point, the parameters will change, and I might make it prettier, or more intuitive. (There is also a nasty global in there, so running more than one at a time is either a feature or a bug, depending on how you use it...)

I will probably gist this at some point and keep it up to date with those changes.