JSON Type Definition, aka RFC8927, is an easy-to-learn, standardized way to define a schema for JSON data. You can use JSON Typedef to portably validate data across programming languages, create dummy data, generate code, and more.
jtd-infer is a tool that generates ("infers") a JSON Typedef schema from
example data.
bash
echo '{ "name": "Joe", "age": 42 }' | jtd-infer | jq
json
{
"properties": {
"age": {
"type": "uint8"
},
"name": {
"type": "string"
}
}
}
On macOS, you can install jtd-infer via Homebrew:
bash
brew install jsontypedef/jsontypedef/jtd-infer
For all other platforms, you can download and extract the binary yourself from
the latest release. You can also install using cargo by running:
bash
cargo install jtd_infer
For high-level guidance on how to use jtd-infer, see "Inferring a JSON
Typedef Schema from Real Data" in the JSON Typedef website docs.
To invoke jtd-infer, you can either:
jtd-infer.jtd-infer reads a sequence of JSON messages. So for example, if you have a
file like this in data.json:
json
{ "name": "john doe", "age": 42 }
{ "name": "jane doe", "age": 45 }
You can give it to jtd-infer in two ways:
```bash
cat data.json | jtd-infer jtd-infer data.json ```
In both cases, you'd get this output:
json
{"properties":{"name":{"type":"string"},"age":{"type":"uint8"}}}
⚠️ This section is often important if you are retrofitting JSON Typedef to a JavaScript-based application.
By default, JSON Typedef will infer the most specific possible type for inputs.
So, for example, it will guess uint8 if it sees a 12 in your input:
bash
echo "12" | jtd-infer
json
{"type":"uint8"}
However, if you're giving JSON Typedef a small sample set, or if you in practice
have data that is far smaller than the actual numerical datatypes your
application supports, then this behavior may be undesirable. For example, it's
common for JavaScript-based applications to actually support float64 for all
numeric inputs, because JavaScript numbers are IEEE double-precision floats.
To tell JSON Typedef to prefer a different type than the one it would normally
guess, you can use --default-number-type to change its behavior. For example:
```bash
#
echo "12" | jtd-infer --default-number-type=float64 ```
json
{"type":"float64"}
Another use-case is if you're writing an application that uses signed 32-bit
ints everywhere, and your example data simply never in practice has examples of
negative numbers or numbers too big for 8- or 16-bit numbers. You can achieve
that by using int32 as your default number type:
bash
echo "12" | jtd-infer --default-number-type=int32
json
{"type":"int32"}
Note that jtd-infer will ignore your default if it doesn't match with the
data. For example, int32 only works with whole numbers, so if a decimal number
or a number too big for 32-bit signed integers comes in, it will fall back to
float64:
```bash
echo "3.14" | jtd-infer --default-number-type=int32 echo "9999999999" | jtd-infer --default-number-type=int32 ```
By default, jtd-infer will never output enum, values, or discriminator
schemas. This is by design: by always being consistent with what it outputs,
jtd-infer is more predictable and reliable.
If you want jtd-infer to output an enum, values, or discriminator, you
can use the --enum-hint, --values-hint, and --discriminator-hint flags.
You can pass each of these flags multiple times.
All of the hint flags accept JSON
Pointers as values. If you're used to the
JavaScript-y syntax of referring to things as $.foo.bar, the equivalent JSON
Pointer is /foo/bar. jtd-infer treats - as a "wildcard". /foo/-/bar is
equivalent to the JavaScript-y $.foo.*.bar.
As a corner-case, if you want to point to the root / top-level of your input,
then use the empty string as the path. See "Using
--values-hint" for an example of this.
--enum-hintBy default, strings are always inferred to be { "type": "string" }:
bash
echo '["foo", "bar", "baz"]' | jtd-infer
json
{"elements":{"type":"string"}}
But you can instead have jtd-infer output an enum by providing a path to the
string you consider to be an enum. In this case, it's any element of the root of
the array -- the JSON Pointer for that is /-:
bash
echo '["foo", "bar", "baz"]' | jtd-infer --enum-hint=/-
json
{"elements":{"enum":["bar","baz","foo"]}}
--values-hintBy default, objects are always assumed to be "structs", and jtd-infer will
generate properties / optionalProperties. For example:
bash
echo '{"x": [1, 2, 3], "y": [4, 5, 6], "z": [7, 8, 9]}' | jtd-infer
json
{"properties":{"y":{"elements":{"type":"uint8"}},"z":{"elements":{"type":"uint8"}},"x":{"elements":{"type":"uint8"}}}}
If your data is more like a map / dictionary, pass a values-hint that points
to the object that you want a values schema from. In this case, that's the
root-level object, which in JSON Pointer is just an empty string:
bash
echo '{"x": [1, 2, 3], "y": [4, 5, 6], "z": [7, 8, 9]}' | jtd-infer --values-hint=
json
{"values":{"elements":{"type":"uint8"}}}
--discriminator-hintBy default, objects are always assumed to be "structs", and jtd-infer will
generate properties / optionalProperties. For example:
bash
echo '[{"type": "s", "value": "foo"},{"type": "n", "value": 3.14}]' | jtd-infer
json
{"elements":{"properties":{"value":{},"type":{"type":"string"}}}}
If your data has a special "type" property that tells you what's in the rest of
the object, then use --discriminator-hint to point to that property.
jtd-infer will output an appropriate discriminator schema instead:
bash
echo '[{"type": "s", "value": "foo"},{"type": "n", "value": 3.14}]' | jtd-infer --discriminator-hint=/-/type | jq
json
{
"elements": {
"discriminator": "type",
"mapping": {
"s": {
"properties": {
"value": {
"type": "string"
}
}
},
"n": {
"properties": {
"value": {
"type": "float64"
}
}
}
}
}
}