Your first transformer

A Jolt transformation has three parts: a source document (the JSON you have), a transformer (JSON describing what you want), and the output Jolt produces.

1. Write a transformer

transformer.json

{
    "name": "#valueOf($.customer.firstName) + ' ' + #valueOf($.customer.lastName)",
    "isActive": "#valueOf($.customer.status) == 'active'",
    "tagCount": "#valueOf($.customer.tags)->#length()"
}

Each string containing an expression is evaluated against the source document. $.customer.firstName is a JSON Path into the source.

2. Run it from C#

These examples use Jolt.Json.Newtonsoft; the Jolt.Json.DotNet API has the same shape.

using Jolt.Json.Newtonsoft;

public string Run(string transformerJson, string sourceJson)
{
    var transformer = JoltJsonTransformer.DefaultWith<TransformerMethods>(transformerJson);
    return transformer.Transform(sourceJson);
}

TransformerMethods is the class holding any C# methods your transformer calls. See external methods for how to write and register them.

3. See the result

Given this source document:

source.json

{
    "customer": {
        "firstName": "Ada",
        "lastName": "Lovelace",
        "status": "active",
        "tags": [ "math", "engines" ]
    }
}

The transformer produces:

output.json

{
    "name": "Ada Lovelace",
    "isActive": true,
    "tagCount": 2
}

Try this example in the playground →

Check your syntax early

Before you have a real source document, Validate() reports syntax and structure problems in the transformer itself. See validation.

Where to go next

• Syntax covers the rules every expression follows.
• Methods and piping explains #method() calls and the -> operator.
• The library reference lists every built-in method.