0x150Lesson 22 of 24

Web APIs, JSON and XML

Talk to web services with Invoke-RestMethod, handle paging and errors, and read, query and edit XML.

30 min 7-question quiz 2 code exercises

By the end of this lesson you can

Call REST APIs with Invoke-RestMethod - GET and POST, headers, query strings and JSON bodies
Handle paging and HTTP errors, and know when to reach for Invoke-WebRequest
Load XML with [xml], query it with dot notation and Select-Xml, edit it, and compare lists with Compare-Object

The Emberfall weather station, the vet’s booking system and the dragon registry all speak HTTP APIs - they answer web requests with JSON instead of web pages. Invoke-RestMethod (alias irm) sends the request and converts the JSON answer into objects, so a web service feels like any other cmdlet.

Most APIs want a few extras: an API key or token in a header, parameters in the query string (?species=wisp&page=2), and for POST/PUT requests a JSON body. Splatting keeps all of that readable.

registry-api.ps1

1$headers = @{ Authorization = "Bearer $env:EMBERFALL_TOKEN" }
2
3# GET: read data
4$dragons = Invoke-RestMethod -Uri 'https://api.emberfall.example/dragons?species=wisp' -Headers $headers
5$dragons | Select-Object name, wingspan
6
7# POST: send data as a JSON body
8$request = @{
9  Uri         = 'https://api.emberfall.example/dragons'
10  Method      = 'Post'
11  Headers     = $headers
12  ContentType = 'application/json'
13  Body        = @{ name = 'Pebble'; species = 'Wyrm'; wingspan = 3 } | ConvertTo-Json
14}
15$created = Invoke-RestMethod @request
16"registered with id $($created.id)"

You can practice all of the parsing without a network: an API response is just JSON text. Build query strings with [uri]::EscapeDataString() so spaces and & don’t break the URL, and use an [ordered] hashtable when the order of fields in a body matters to you.

offline-response.ps1

1$response = @'
2{ "page": 1, "dragons": [
3  { "name": "Skyla", "wingspan": 15, "tags": ["storm", "fast"] },
4  { "name": "Glim", "wingspan": 1, "tags": [] }
5] }
6'@ | ConvertFrom-Json
7$response.dragons.Count
8$response.dragons[0].tags -join ', '
9$response.dragons | Sort-Object wingspan | ForEach-Object name
10$search = 'fire & ice'
11"https://api.emberfall.example/search?q=$([uri]::EscapeDataString($search))"
12[ordered]@{ name = 'Pebble'; wingspan = 3 } | ConvertTo-Json -Compress

Output

2
storm, fast
Glim
Skyla
https://api.emberfall.example/search?q=fire%20%26%20ice
{"name":"Pebble","wingspan":3}

Paging and errors

APIs rarely return everything at once. They send a page and tell you where the next one is - a next link, or a page number to increase. Loop until there’s no next page.

When the server answers with an error status (404 Not Found, 401 Unauthorized, 500 Server Error), Invoke-RestMethod throws a terminating error - so try/catch works, and $_.Exception.Response.StatusCode tells you which. Follow a request and its errors step by step:

Try it

Follow the requests

Step through a script that fetches every page of dragons, then looks up one that doesn’t exist. Predict before each reveal.

Message 1 of 9Predicted 0/0

Your script

Invoke-RestMethod

api.emberfall.example

paging.ps1

1$all = [System.Collections.Generic.List[object]]::new()
2$uri = 'https://api.emberfall.example/dragons?page=1'
3while ($uri) {
4  $page = Invoke-RestMethod -Uri $uri -Headers $headers
5  $all.AddRange(@($page.dragons))
6  $uri = if ($page.next) { "https://api.emberfall.example$($page.next)" }
7}
8"fetched $($all.Count) dragons"
9
10try {
11  Invoke-RestMethod -Uri 'https://api.emberfall.example/dragons/smaug' -Headers $headers
12} catch {
13  $status = [int]$_.Exception.Response.StatusCode
14  if ($status -eq 404) { 'No such dragon - carry on' } else { throw }
15}

XML

Older systems, config files (web.config, .csproj), RSS feeds and many government datasets speak XML. Cast text with [xml] and PowerShell gives you an XmlDocument you can walk with dots: elements and attributes both become properties.

For searching, Select-Xml takes an XPath query: //dragon means “every dragon element anywhere”, [@species="Wisp"] filters by attribute. Edit with CreateElement(), SetAttribute() and AppendChild(), then .Save($path) (use a full path).

census.ps1

1[xml]$census = @'
2<census date="2026-03-14">
3  <dragon name="Ember" species="Firedrake" wingspan="12" />
4  <dragon name="Glim" species="Wisp" wingspan="1" />
5  <dragon name="Skyla" species="Stormwing" wingspan="15" />
6</census>
7'@
8$census.census.date
9$census.census.dragon.Count
10$census.census.dragon | Where-Object { [int]$_.wingspan -gt 10 } | ForEach-Object name
11(Select-Xml -Xml $census -XPath '//dragon[@species="Wisp"]').Node.name
12($census.census.dragon | Measure-Object -Property wingspan -Sum).Sum
13$pebble = $census.CreateElement('dragon')
14$pebble.SetAttribute('name', 'Pebble')
15[void]$census.census.AppendChild($pebble)
16$census.census.dragon.Count

Output

2026-03-14
3
Ember
Skyla
Glim
28
4

Compare-Object answers “what changed between these two lists?” - perfect for comparing yesterday’s census with today’s. <= means only in the first (reference) list, => only in the second (difference) list, and -IncludeEqual adds == for items in both.

compare.ps1

1$monday = 'Ember', 'Glim', 'Skyla', 'Pebble'
2$tuesday = 'Ember', 'Skyla', 'Pebble', 'Nimbus'
3Compare-Object -ReferenceObject $monday -DifferenceObject $tuesday |
4  ForEach-Object { "$($_.InputObject) $($_.SideIndicator)" }

Output

Nimbus =>
Glim <=

Key takeaways

Invoke-RestMethod sends HTTP requests and turns JSON replies into objects; splat Uri, Method, Headers, ContentType and Body.
Escape query values with [uri]::EscapeDataString(); loop for pages; error statuses throw, so use try/catch.
Keep tokens in environment variables or a secret vault, never in the script.
[xml] plus dot notation reads XML, Select-Xml queries it with XPath, and Compare-Object shows what changed between two lists.

Lesson quiz

7 questions · pass with 5 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write PowerShell scripts

Write a script in the editor and run it for real against sample input, which is piped into your script as $input. Scripts run on PowerShell 6.2 through Try It Online (tio.run), a free public service, so these exercises avoid PowerShell 7-only syntax; your script and test input are sent there.

Exercise 1

Read an API page

+25 XP

The input is one page of JSON from the dragon registry, with page, totalPages, next (a URL or null) and a dragons array of objects with name, wingspan and status.

Print page 1 of 2, then each dragon as Skyla (15 m) - flying, widest wingspan first, then next page: <url> - or last page when next is null. Read the whole input with $input | Out-String.

First page
Last page

script.ps1

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Lessons teach PowerShell 7; your script runs on PowerShell 6.2 via Try It Online (tio.run), a free public service, so stick to syntax that works there. Test input is piped into your script as $input. Your script and test input are sent to that service.

Exercise 2

Who came, who went?

+25 XP

The input is an XML census with two <roster> elements, day="monday" and day="tuesday", each listing <dragon name="..."/> elements.

Use Compare-Object to print arrived: <names> (on Tuesday only), left: <names> (on Monday only) - sorted, comma-separated, or nobody - and unchanged: <n>.

One in, one out
Visitors only

script.ps1

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