Return values enable PowerShell functions to send data back to the caller when execution completes. This guide dives deep into return value concepts, real-world usage, best practices, troubleshooting and more for mastering function output.

What Exactly Are Return Values?

A return value allows a PowerShell function to pass computed results, status information, data structures and more back to the calling code:

Return Value Diagram

Some key capabilities unlocked by return values:

  • Outputting Results – Return computational outcomes, processed data
  • Error Handling – Indicate failures via boolean flags
  • Debugging – Insight into a function‘s internal state
  • Early Exits – Abort further processing prematurely
  • Complex Objects – Return rich nested data structures

Here is a simple example:

# Function performs computation
function Get-Square($num) {
  $square = $num * $num
  return $square
}

# Caller receives result
$result = Get-Square 5
Write-Output $result # Prints 25

The return value gets passed back to the assigned variable $result for further usage.

Return Value Terminology

Some common terms used:

  • Return Type: The .NET type of data returned – string, int, bool etc.
  • Return Value: The actual data returning from the function.
  • Return Statement: Statement used to trigger return – return $someValue

Key Return Value Usage Scenarios

Let‘s explore some of the most popular use cases for return values:

1. Returning Computational Results

Functions commonly perform calculations, process input data and return the outcome: 

function Get-Factorial($num) {
  $result = 1
  for ($i = 1; $i -le $num; $i++) {    
    $result *= $i
  }

  return $result
}

$fact5 = Get-Factorial 5
# $fact5 contains 120

Here the return value enables retrieving the factorial computation result for subsequent usage.

Some other examples:

  • Format a string and return the updated string
  • Query a database and return processed dataset
  • Calculate shopping cart cost after discounts and taxes

2. Error Handling via Return Codes

Return values can indicate a function‘s success or failure status:

function Backup-Data([string]$path) {
  try {
    # Logic to backup data
  } 
  catch {
    # Failed
    return $false
  }

  return $true # Succeeded
}

$result = Backup-Data -path .\Data
if (-not $result) {
  Write-Host "Backup failed!"
} else {
  Write-Host "Backup succeeded" 
}

The boolean return indicates whether the backup operation succeeded. The caller then takes appropriate error handling actions based on this status.

Common conventions:

  • Return $true for success
  • Return $false for failure

Some other examples:

  • Validate user credentials passed to a function
  • Indicate if a file was accessed/written successfully
  • Inform whether an API call returned valid data

3. Aborting Function Execution Early

Return statements abort any further code execution inside a function:

function Deploy-Application($env) {

  if ($env -ne "prod") {
    Write-Host "Only prod deployments allowed!"
    return # Early exit
  } 

  Write-Host "Starting production deployment..."

  # Remainder of complex deployment logic
}

Here if a non-prod environment is passed, the function exits early without running unnecessary deployment steps.

Some other examples:

  • Exit if invalid user input passed
  • Skip cache population if already cached
  • Break loop execution on specific counting scenarios

Returns vs Write-Output

Write-Output also outputs data but does NOT exit the function. Returns additionally abort further processing.

4. Encapsulating Status in Complex Objects

Return rich error objects with metadata instead of mere booleans:

function Get-LogData() {

  try {
    # Logic to query logs
  }
  catch {

    $errDetails = [PSCustomObject]@{
      Success = $false
      Message = $_.Exception.Message  
    }

    return $errDetails

  }

  $logs = # Fetched log data

  $result = [PSCustomObject]@{
    Success = $true
    Logs = $logs
  }

  return $result

}

$result = Get-LogData

if (-not $result.Success) {
  Write-Host $result.Message
} else {
  # Process log data
}

This approach encapsulates both status and additional context in a single return value object. The caller can elegantly branch based on the structured result.

5. Returning Data Structures

Return entire data structures like arrays/hashes instead of discrete elements:

function Get-TopProductSales() {

  $topSales = @()

  # Logic to query top yearly sales

  foreach ($sale in $topSalesData) {    
    $topSales += [PSCustomObject]@{
      Product = $sale.Product
      Revenue = $sale.Revenue      
    }
  }  

  return $topSales

}

$topProducts = Get-TopProductSales
# Array returned for further processing

This enables greater post-processing of composite data structures.

Some other examples:

  • Return database resultsets
  • Return lists of aggregated calculation outputs
  • Return hashtable/dictionary containing processed state

Working with Return Values

Now let‘s explore patterns for accessing return data.

Approach 1: Storing in Variable

Store function output by assigning to variable: 

$processed = Function-ToProcessData

Key points:

  • Use descriptive variable names
  • Operate on return value like any other variable
  • Watch out for scope when declaring variables

Works best for:

  • Reusing returned data in further logic
  • Processing result with native tools like Where-Object

Approach 2: Piping to other Commands

Pipe return value to other commands:

Function-ToProcessData | Export-Csv -Path parsed.csv

Key points:

  • Leverage PowerShell‘s pipeline strengths
  • Avoid intermediate variables
  • Allow further processing in pipeline

Works best for:

  • Streaming tabular data to other cmdlets
  • Chaining multiple transformations
  • Outputting data to external sinks

Approach 3: Using directly

Call functions inline without capturing return value:

Write-Host "Result was $(Function-ToProcessData)"

Key points:

  • Return value gets printed to host
  • No variable required
  • Hard to reuse result

Works best for:

  • Quick inspecting values in host
  • Simple logging / outputting
  • Code clarity in scripts

Now let‘s look at some key nuances when working with function returns.

Working Effectively With Return Values

Return Single Elements vs Entire Streams

By default return values output as single discrete objects back to the caller.

To return an entire stream of objects, ensure your function‘s OutputType attribute is set, typically to System.Collections.ArrayList:

function Get-LogData {
  [OutputType([System.Collections.ArrayList])]  
  param() 

  # Logic  
}

This allows returning streams for piping across multiple commands.

Accessing Return Values in Calling Scope

Return values flow back to the caller‘s scope:

$x = 100

function Test {
   $x = 50
   return $x
}

$val = Test
$val # Contains 50
$x # Still contains 100

The function scope does not overwrite $x in caller scope.

Data Type Conversion Warnings

If return value types don‘t match assigned variables, coercion warnings may appear:

function Test {
  return "demo"
}

$x = Test

Cannot convert value "demo" to type "System.Int32". Error: "Input string was not in a correct format."

Where possible declare explicit data types to avoid this.

Some other best practices:

  • Only return data actually needed
  • Avoid side-effects outside function scope
  • Validate input parameters at start

Comparing Return Values & Write Pipeline

  • Return passes discrete value back to caller.
  • Write-Output streams objects through the pipeline.

When to use Return:

  • Need result for further computation
  • Require status codes from execution
  • Desire simple programmatic control flow

When to use Write-Pipeline:

  • Stream large datasets to other cmdlets
  • Output to external systems like files
  • Chaining multiple processing steps

Here is an example with both approaches:

function Process-Data() {

  # Initial logic  

  $status = [PSCustomObject] @{
    Success = $true
  }

  Write-Output $status

  # Further logic

  return $finalResult
}

$resultVar = Process-Data

Process-Data | Export-Csv -Path output.csv

Write-Output allows streaming interim data, while return passes the final result.

Avoiding Common Return Statement Pitfalls

Let‘s explore some poor practices that should be dodged:

  • Not using returns at all – Limits error handling abilities
  • Returning unused interim values – Causes performance overhead
  • No validation before return – Outputs bogus data
  • Returning ambiguous status codes – Breaks caller logic
  • Overusing return statements – Reduces code clarity

Where possible adopt these best practices:

  • Validate parameters at start of functions
  • Only return data required by caller
  • Pick clear success vs failure indicators
  • Limit usage for clarity and ease debugging
  • Always handle errors gracefully

Adopting these will maximize robustness.

Real World Function Examples

Let‘s look at some realistic examples:

1. Excel Report Generation

function Build-ExcelReport() {

  $excel = New-Object -ComObject Excel.Application  
  $workbook = $excel.Workbooks.Add()

  try {

    # Populate sheet with data    

    $workbook.SaveAs("C:\Reports\report.xlsx")

    return $true # Report built

  }
  catch {

    return $false # Failed

  }
  finally {

    $excel.Quit()

  }

}

$result = Build-ExcelReport
if (-not $result) {
  Write-Host "Error generating report"
}

Here return value indicates success/failure in building the file.

2. REST API Data Fetching

function Get-Users() {

  $response = Invoke-RestMethod https://api.server.com/users

  if ($response.status -ne 200) {
    return $false
  }

  $users = $response.data

  return $users

}

$users = Get-Users
if (-not $users) {
  Write-Host "Error fetching users"  
} else {
  # Process $users array
}

This uses returns to flag API errors before returning parsed user data.

3. Database Query Wrapper

function Get-SalesData() {

  try {

    $sql = "SELECT * FROM sales" 
    $connection = New-Object System.Data.SqlClient.SqlConnection
    $connection.ConnectionString = "Server=mysql;Database=store;Integrated Security=True"

    $command = New-Object System.Data.SqlClient.SqlCommand
    $command.CommandText = $sql
    $command.Connection = $connection 

    $connection.Open()
    $sales = $command.ExecuteReader() 

    return $sales

  } catch {

    return $false

  }

}

$result = Get-SalesData
if (-not $result) {
  Write-Host "Error querying database"
} else {
  # Process queried data 
}

Again returns gracefully handle query errors.

Summary

Key highlights:

  • Return values provide a mechanism for functions to output data to callers.
  • They have many uses like computational output, error flagging, early exits etc.
  • Variables and pipelines offer ways to capture return data.
  • Certain nuances exist around streaming, scopes, data types.
  • Contrast with Write-Output for alternative streaming.
  • Follow best practices to avoid common return issues.
  • Usage in the real-world brings together major concepts.

With robust return value handling, reusable functional PowerShell scripts can be built safely.

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