A JSON templating engine that

Sometimes you have a json-based REST API, or have to encode data as JSON to pass it as part of a larger object. However, you might also want the possibility of using custom variables in the json, or have basic templating ability. The JSON JSON template is a minimal templating language that is also valid json itself, so a full template can be parsed and passed as JSON without losing its expressivity.

The example given below will use the following values as context in its template:

{ "foo": true, "bar": 12, "baz": {"hello": "world","tpl": "templating"}, "arr": [1,false,"hi"], "a": "b", "f": (v) => v*2 }

(() => represents a javascript arrow function)

{ "hi!": "normal json is normal json", "but": "once you add the {{ baz.tpl }} ability", "you": "can do many things like calling functions: {{ f(bar) + 1 }}", "basic": "access is similar to javascript template strings: {{ arr[2] }}", "{{ a }}": "keys can be templated too, and overwrite constant keys", "b": "like this one", "what about non-strings?": [ "$ before template value is interpreted as json instead of string", "${{ bar }}", "{{ bar }}" ], "The value of this key uses spread syntax (...), telling the template system to directly add the given keys from the attached object, ignoring the key value.": "${{ ...baz }}", "likewise": ["you can extend arrays in the same way", "${{ ...arr }}"], "${{ arr[0]==1 }}": { "keys": "can be added conditionally by setting object key to a boolean", "and": "you can also do basic conditionals like in javascript: {{ bar >= 12 ? 'yes!' : 'no' }}" }, "defaults": "can be set using | {{ not_defined | baz['neither is this'] | 'lol' }}", "finally": [ "arrays are pythonic", "${{ arr[:-1] }}", "and spread syntax (...) is supported everywhere", "${{ [...arr,...arr] }}", "and", "${{ {...baz, 'tpl': 'newvalue'} }}" ] } Output

{ "hi!": "normal json is normal json", "but": "once you add the templating ability", "you": "can do many things like calling functions: 25", "basic": "access is similar to javascript template strings: hi", "b": "keys can be templated too, and overwrite constant keys", "what about non-strings?": [ "$ before template value is interpreted as json instead of string", 12, "12" ], "hello": "world", "tpl": "templating", "likewise": ["you can extend arrays in the same way", 1, false, "hi"], "keys": "can be added conditionally by setting object key to a boolean", "and": "you can also do basic conditionals like in javascript: yes!", "defaults": "can be set using | lol", "finally": [ "arrays are pythonic", [1, false], "and spread syntax (...) is supported everywhere", [1, false, "hi", 1, false, "hi"], "and", { "hello": "world", "tpl": "newvalue" } ] } Prior Work

This templating language joins several existing approaches:

JsonJsonTemplate supports more general operations than these approaches, at the cost of slightly higher complexity.

The JavaScript implementation of the template system generates a function that applies the template to a given context object. It can be used as follows:

import jjt from "json-json-template"; const input = JSON.parse(jsonstring); const context = { foo: true, bar: 12, baz: { hello: "world", tpl: "templating" }, arr: [1, false, "hi"], a: "b", f: (v) => v * 2, }; // Generate the template function const template = jjt(input); // Run it on the context object, giving the resulting object. const output = template(context); console.log(JSON.stringify(output)); // You can also see the code that was generated for the template with the src property: console.log("template code:", template.src);