JSON Filtering - Changedetection.io

JSON filtering allows you to extract specific values from JSON responses, whether from APIs or JSON embedded in HTML pages. changedetection.io supports both JSONPath and jq for flexible JSON querying.

How JSON Filtering Works

When you apply a JSON filter, changedetection.io:

Fetches the content from the URL
Parses the JSON (either pure JSON or extracts it from HTML)
Applies your filter to extract specific values
Formats the output for monitoring
Detects changes in the extracted data

Key Benefits:

Monitor API responses for changes
Extract nested data structures
Filter and transform JSON data
Parse JSON embedded in HTML (like LD+JSON)
Use logical operations to filter results

Filtering Methods

changedetection.io supports three JSON filtering syntaxes:

JSONPath
jq
jq (Raw Output)

Prefix: json:Library: jsonpath-ngBest for: Simple to moderate JSON queriesExample:

json:$.products[*].price
json:$..author
json:$.store.book[0].title

Prefix: jq:Library: jq (full implementation)Best for: Complex transformations and logicExample:

jq:.products[] | select(.price < 100) | .name
jq:.items | map(.title)
jq:.data.users[].email

jq is not available on all platforms (requires compilation on Windows). Use JSONPath as a cross-platform alternative.

Prefix: jqraw:Difference: Returns raw values without JSON formattingExample:

jqraw:.price

Returns: 29.99 instead of "29.99"

JSONPath Syntax

Basic Selection

json:$.store.book[0].title

Access nested properties with dot notation.

json:$.products[*].name

Select all names from products array.

json:$..price

Recursive descent - finds all price fields at any level.

Array Access

json:$.items[0]

First item in array.

json:$.items[-1]

Last item in array.

json:$.items[0:3]

Slice - first three items.

Filtering

json:$.products[?(@.price < 100)]

Filter products where price is less than 100.

json:$.books[?(@.author == 'Tolkien')]

Filter books by author.

jq Syntax

Basic Selection

jq:.store.book[0].title

Access nested properties.

jq:.products[].price

Extract all prices from products array.

Filtering with select()

jq:.products[] | select(.price < 100)

Filter products under $100.

jq:.items[] | select(.stock > 0) | .name

Get names of items in stock.

Transformations

jq:.products | map(.price)

Extract array of all prices.

jq:.users | map({name, email})

Create new objects with selected fields.

jq:.prices | add

Sum all prices.

Practical Examples

Monitor API Price

{
  "product": {
    "id": 12345,
    "name": "Gaming Laptop",
    "price": 1299.99,
    "currency": "USD",
    "stock": 5
  }
}

json:$.product.price

jq:.product.price

1299.99

Extract Multiple Fields

JSONPath
jq

json:$.product.name
json:$.product.price
json:$.product.stock

Each filter on a new line extracts different fields.

jq:.product | "\(.name): $\(.price) (Stock: \(.stock))"

Formats output as: Gaming Laptop: $1299.99 (Stock: 5)

Monitor Array of Items

{
  "products": [
    {"name": "Laptop", "price": 999},
    {"name": "Mouse", "price": 29},
    {"name": "Keyboard", "price": 79}
  ]
}

json:$.products[*].name

json:$.products[0].price

jq:.products[] | select(.price < 100) | .name

Embedded JSON in HTML

changedetection.io can automatically extract and parse JSON embedded in HTML pages, particularly useful for:

LD+JSON (Linked Data JSON) - Structured data for SEO
Application State - JavaScript state stored in script tags
API Data - JSON embedded for client-side rendering

LD+JSON Product Data

<html>
<head>
  <script type="application/ld+json">
  {
    "@context": "http://schema.org",
    "@type": "Product",
    "name": "Gaming Laptop",
    "offers": {
      "@type": "Offer",
      "price": "1299.99",
      "priceCurrency": "USD",
      "availability": "http://schema.org/InStock"
    }
  }
  </script>
</head>
<body>...</body>
</html>

json:$.offers.price

json:$.offers.priceCurrency

"1299.99"
"USD"

changedetection.io automatically detects and parses <script type="application/ld+json"> tags. Just use your JSON filter as if you were querying the JSON directly.

Multiple JSON Blocks

If the HTML contains multiple JSON blocks:

<script type="application/ld+json">{...}</script>
<script type="application/ld+json">{...}</script>

changedetection.io will search through all blocks and return the first matching result.

Advanced Filtering

Filter by Multiple Conditions (jq)

jq:.products[] | select(.price < 100 and .stock > 0) | .name

Finds products under $100 that are in stock.

jq:.items[] | select(.category == "electronics" or .category == "computers")

Filters items by multiple categories.

Nested Data Extraction

json:$.store.books[*].author.name

Navigates through nested objects.

jq:.data.users[].profile.email

Accesses deeply nested fields.

Array Transformations (jq)

jq:.products | map(.price) | add / length

Calculates average price.

jq:.items | sort_by(.price) | .[0]

Finds cheapest item.

jq:.products | group_by(.category) | map({category: .[0].category, count: length})

Groups and counts by category.

Conditional Output (jq)

jq:if .stock > 0 then "In Stock" else "Out of Stock" end

Returns different values based on conditions.

jq:.products[] | if .price < 50 then .name else empty end

Only outputs names of products under $50.

Output Formatting

Single Value

When your filter returns a single value:

json:$.price

Output: 29.99 (unquoted number)

Multiple Values

When your filter returns multiple values:

json:$.products[*].price

Output:

String Values

json:$.product.name

Output: "Gaming Laptop" (includes quotes) Use jqraw: prefix to get unquoted strings:

jqraw:.product.name

Output: Gaming Laptop

Testing JSON Filters

Using Online Tools

JSONPath:

JSONPath Online Evaluator
Paste your JSON and test expressions

jq:

jq play
Interactive jq playground

Using Browser Console

Open DevTools Console (F12)

Fetch and test your JSON:

fetch('https://api.example.com/data')
  .then(r => r.json())
  .then(data => console.log(data))

Examine structure and test paths

Common Patterns

Pattern: Extract Nested Price

json:$.offers.price
json:$..price

Use case: Product pricing from structured data.

Pattern: Monitor Stock Status

json:$.availability

Use case: Track product availability.

Pattern: Track Multiple Products

json:$.products[*].name
json:$.products[*].price

Use case: Monitor product catalog.

Pattern: Filter Price Range (jq)

jq:.products[] | select(.price > 50 and .price < 200)

Use case: Find mid-range products.

Pattern: Count Items (jq)

jq:.items | length

Use case: Track number of available items.

Common Pitfalls

Pitfall #1: Forgetting the Prefix

$.products[*].price  # Wrong - will be treated as CSS selector

Correct:

json:$.products[*].price

Pitfall #2: Path Not FoundIf your filter returns nothing:

Verify JSON structure (use browser DevTools)
Check for typos in property names
Ensure arrays use correct syntax: [*] or []
Try recursive descent: $..property_name

Pitfall #3: Wrong Filter Type

json:$.products[]  # JSONPath doesn't use [] for iteration

Use jq instead:

jq:.products[]

Or JSONPath:

json:$.products[*]

Pitfall #4: jq Not Availablejq requires compilation and may not be available on Windows.Fallback: Use JSONPath for cross-platform compatibility:

json:$.products[?(@.price < 100)].name

When to Use JSON Filtering

Good for:

Monitoring REST APIs
Tracking prices from JSON endpoints
Extracting structured data from HTML (LD+JSON)
Processing JSON with logical filtering
Monitoring nested data structures

Not ideal for:

HTML content (use CSS selectors or XPath)
XML/RSS feeds (use XPath)
Plain text content

JSONPath vs jq

Feature	JSONPath	jq
Syntax complexity	Simpler	More powerful
Platform support	✅ All platforms	❌ May not work on Windows
Filtering	Basic `[?(@.x)]`	Advanced `select()`
Transformations	Limited	Extensive (map, group, etc.)
Learning curve	Easier	Steeper
Output formatting	Automatic	Full control
Math operations	❌ No	✅ Yes (add, multiply, etc.)

Real-World Examples

Example: Monitor GitHub API

jq:.stargazers_count

Track repository stars from GitHub API.

jq:.open_issues_count

Monitor open issues count.

Example: Track Cryptocurrency Price

json:$.bitcoin.usd

Extract Bitcoin price in USD from crypto API.

jq:.data.quotes.USD.price

Alternative structure from different API.

Example: Monitor E-commerce Inventory

jq:.products[] | select(.stock < 10) | "\(.name): \(.stock) left"

Alerts when stock is low (under 10 units).

Example: Track Weather Data

json:$.current.temp_c
json:$.current.condition.text

Extract temperature and conditions from weather API.

Example: Monitor Job Postings API

jq:.jobs[] | select(.location == "Remote") | .title

Filter remote job titles from job board API.

Combining with Other Filters

You can combine JSON filtering with other features:

Ignore Text

After extracting JSON data, filter out unwanted values: Filter: json:$.products[*].name Ignore text: Discontinued

Trigger Keywords

Trigger only when specific values appear: Filter: json:$.status Trigger text: Available, In Stock

Extract Text (Regex)

Further process extracted JSON values: Filter: json:$.price Extract text: /\d+\.\d{2}/ (extract just the number)

Debugging Tips

No Output

Verify URL returns valid JSON (check in browser)
Test filter in online JSONPath/jq playground
Check for typos in property names
Try recursive descent: $..property
Enable browser-based fetching if JSON is loaded dynamically

Unexpected Output Format

Single value returns without array brackets
Multiple values return as JSON array
Use jqraw: for unquoted string output
Check if your path is too broad ($..price finds ALL prices)

Parsing Errors

Ensure content-type is JSON or contains valid JSON
Check for BOM (Byte Order Mark) issues
Verify JSON is not wrapped in extra markup
For HTML-embedded JSON, ensure <script type="application/ld+json"> is correct

CSS Selectors - Extract HTML content
XPath - Query XML and RSS feeds
API Monitoring - Best practices for monitoring APIs

​How JSON Filtering Works

​Filtering Methods

​JSONPath Syntax

​Basic Selection

​Array Access

​Filtering

​jq Syntax

​Basic Selection

​Filtering with select()

​Transformations

​Practical Examples

​Monitor API Price

​Extract Multiple Fields

​Monitor Array of Items

​Embedded JSON in HTML

​LD+JSON Product Data

​Multiple JSON Blocks

​Advanced Filtering

​Output Formatting

​Single Value

​Multiple Values

​String Values

​Testing JSON Filters

​Using Online Tools

​Using Browser Console

​Common Patterns

​Pattern: Extract Nested Price

​Pattern: Monitor Stock Status

​Pattern: Track Multiple Products

​Pattern: Filter Price Range (jq)

​Pattern: Count Items (jq)

​Common Pitfalls

​When to Use JSON Filtering

​JSONPath vs jq

​Real-World Examples

​Combining with Other Filters

​Ignore Text

​Trigger Keywords

​Extract Text (Regex)

​Debugging Tips

​No Output

​Unexpected Output Format

​Parsing Errors

​Related Topics

How JSON Filtering Works

Filtering Methods

JSONPath Syntax

Basic Selection

Array Access

Filtering

jq Syntax

Basic Selection

Filtering with select()

Transformations

Practical Examples

Monitor API Price

Extract Multiple Fields

Monitor Array of Items

Embedded JSON in HTML

LD+JSON Product Data

Multiple JSON Blocks

Advanced Filtering

Output Formatting

Single Value

Multiple Values

String Values

Testing JSON Filters

Using Online Tools

Using Browser Console

Common Patterns

Pattern: Extract Nested Price

Pattern: Monitor Stock Status

Pattern: Track Multiple Products

Pattern: Filter Price Range (jq)

Pattern: Count Items (jq)

Common Pitfalls

When to Use JSON Filtering

JSONPath vs jq

Real-World Examples

Combining with Other Filters

Ignore Text

Trigger Keywords

Extract Text (Regex)

Debugging Tips

No Output

Unexpected Output Format

Parsing Errors

Related Topics