TiddlyWiki5/editions/tw5.com/tiddlers/filters/jsonextract.tid

created: 20220611104737314
modified: 20220611104737314
tags: [[Filter Operators]] [[JSON Operators]]
title: jsonextract Operator
caption: jsonextract
op-purpose: retrieve the JSON string of a property from JSON strings
op-input: a selection of JSON strings
op-parameter: one or more indexes of the property to retrieve
op-output: the JSON string values of each of the retrieved properties

<<.from-version "5.2.4">> See [[JSON in TiddlyWiki]] for background.

The <<.op jsonextract>> operator is used to retrieve values from JSON data as JSON substrings. See also the following related operators:

* <<.olink jsonget>> to retrieve the values of a property in JSON data
* <<.olink jsontype>> to retrieve the type of a JSON value
* <<.olink jsonindexes>> to retrieve the names of the fields of a JSON object, or the indexes of a JSON array

Properties within a JSON object are identified by a sequence of indexes. In the following example, the value at `[a]` is `one`, and the value at `[d][f][0]` is `five`.

```
{
    "a": "one",
    "b": "",
    "c": "three",
    "d": {
        "e": "four",
        "f": [
            "five",
            "six",
            true,
            false,
            null
        ],
        "g": {
            "x": "max",
            "y": "may",
            "z": "maize"
        }
    }
}
```

The following examples assume that this JSON data is contained in a variable called `jsondata`.

The <<.op jsonextract>> operator uses multiple parameters to specify the indexes of the property to retrieve. Values are returned as literal JSON strings:

```
[<jsondata>jsonextract[a]] --> "one"
[<jsondata>jsonextract[d],[e]] --> "four"
[<jsondata>jsonextract[d],[f],[0]] --> "five"
[<jsondata>jsonextract[d],[f]] --> ["five","six",true,false,null]
[<jsondata>jsonextract[d],[g]] --> {"x":"max","y":"may","z":"maize"}
```

<<.from-version "5.3.2">> Negative indexes into an array are counted from the end, so -1 means the last item, -2 the next-to-last item, and so on:

```
[<jsondata>jsonextract[d],[f],[-1]] --> null
[<jsondata>jsonextract[d],[f],[-2]] --> false
[<jsondata>jsonextract[d],[f],[-4]] --> "six"
```

Indexes can be dynamically composed from variables and transclusions:

```
[<jsondata>jsonextract<variable>,{!!field},[0]]
```

A subtlety is that the special case of a single blank parameter is used to identify the root object. Thus:

```
[<jsondata>jsonextract[]] --> {"a":"one","b":"","c":"three","d":{"e":"four","f":["five","six",true,false,null],"g":{"x":"max","y":"may","z":"maize"}}}
```