75 lines
2.5 KiB
Plaintext
75 lines
2.5 KiB
Plaintext
created: 20220611104737314
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modified: 20220611104737314
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tags: [[Filter Operators]] [[JSON Operators]]
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title: jsonextract Operator
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caption: jsonextract
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op-purpose: retrieve the JSON string of a property from JSON strings
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op-input: a selection of JSON strings
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op-parameter: one or more indexes of the property to retrieve
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op-output: the JSON string values of each of the retrieved properties
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<<.from-version "5.2.4">> See [[JSON in TiddlyWiki]] for background.
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The <<.op jsonextract>> operator is used to retrieve values from JSON data as JSON substrings. See also the following related operators:
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* <<.olink jsonget>> to retrieve the values of a property in JSON data
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* <<.olink jsontype>> to retrieve the type of a JSON value
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* <<.olink jsonindexes>> to retrieve the names of the fields of a JSON object, or the indexes of a JSON array
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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`.
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```
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{
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"a": "one",
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"b": "",
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"c": "three",
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"d": {
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"e": "four",
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"f": [
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"five",
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"six",
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true,
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false,
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null
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],
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"g": {
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"x": "max",
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"y": "may",
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"z": "maize"
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}
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}
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}
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```
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The following examples assume that this JSON data is contained in a variable called `jsondata`.
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The <<.op jsonextract>> operator uses multiple parameters to specify the indexes of the property to retrieve. Values are returned as literal JSON strings:
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```
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[<jsondata>jsonextract[a]] --> "one"
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[<jsondata>jsonextract[d],[e]] --> "four"
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[<jsondata>jsonextract[d],[f],[0]] --> "five"
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[<jsondata>jsonextract[d],[f]] --> ["five","six",true,false,null]
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[<jsondata>jsonextract[d],[g]] --> {"x":"max","y":"may","z":"maize"}
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```
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<<.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:
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```
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[<jsondata>jsonextract[d],[f],[-1]] --> null
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[<jsondata>jsonextract[d],[f],[-2]] --> false
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[<jsondata>jsonextract[d],[f],[-4]] --> "six"
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```
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Indexes can be dynamically composed from variables and transclusions:
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```
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[<jsondata>jsonextract<variable>,{!!field},[0]]
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```
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A subtlety is that the special case of a single blank parameter is used to identify the root object. Thus:
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```
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[<jsondata>jsonextract[]] --> {"a":"one","b":"","c":"three","d":{"e":"four","f":["five","six",true,false,null],"g":{"x":"max","y":"may","z":"maize"}}}
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```
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