factorialExample.ps1

# Simple approach - let PS's type conversion logic handle it.
# Pitfall: someone can just supply arbitrary text to the function, which is a bit unfortunate.

function Get-Factorial {
    param($Number)
    
    if ($Number -eq 0) {
        1
    }
    else {
        $Number * (Get-Factorial -Number ($Number - 1))
    }
}

# More robust approach: type cast the parameter itself
# Pitfall: No handling of negative numbers, will cause infinite loops

function Get-Factorial {
    param(
        [int]
        $Number
    )
    
    if ($Number -eq 0) {
        1
    }
    else {
        $Number * (Get-Factorial -Number ($Number - 1))
    }
}

# Strict approach: prevent negatives with parameter attributes,
# and prevent extra values being passed in arbitrarily with [CmdletBinding()]

function Get-Factorial {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory, Position = 0)]
        [ValidateRange(0, [int]::MaxValue)]
        [int]
        $Number
    )
    
    if ($Number -eq 0) {
        1
    }
    else {
        $Number * (Get-Factorial -Number ($Number - 1))
    }
}