Lesson 9

JSONPath Queries

Extract values from nested documents using path expressions.

When a JSON document is large, you often need one field without mentally traversing every level. JSONPath is a query language—similar in spirit to XPath for XML—for selecting nodes in JSON trees.

Sample document

{
  "store": {
    "books": [
      { "title": "JSON 101", "price": 12.5 },
      { "title": "API Design", "price": 28 }
    ]
  }
}

Common expressions

Expression	Meaning
`$`	Root of the document
`$.store.books`	The `books` array
`$.store.books[0].title`	Title of first book
`$.store.books[*].price`	All prices
`$..price`	`price` anywhere under root

Read $ as “start here,” . as “child property,” [n] as array index, [*] as every element, and .. as recursive descent.

Use cases

Pull a user ID from a nested API response in tests or scripts
Filter log events where level == "error" across heterogeneous payloads
Document “where to find” fields in integration guides

JSONPath vs manual code

In JavaScript you might write data.store.books[0].title. JSONPath shines when:

Depth varies between API versions
You query the same path in config across many files
Tools expose a path box for ad-hoc inspection

Limitations

Implementations differ slightly on extensions (filters, slices). For production pipelines, pin a library version and add tests for critical paths. JSONPath reads structure—it does not fix invalid JSON; parse and validate first.

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