Syntax

A transformer is ordinary JSON. Jolt looks for expressions inside its property names and string values, evaluates them against the source document, and writes the results to the output.

The building blocks

Token	Meaning	Example
#name(...)	A method call. Names are case-sensitive.	#valueOf($.a)
$...	A JSON Path into the source document	$.order.items
@name	A range variable	@total
'text'	A string literal	'hello'
->	Pipe a value into a method’s first parameter	@x->#length()
@x: ...	A lambda	@x: @x.price > 10
a..b, ^n	A range	@text[1..3]
into	Names the output of a property-name construct	#foreach(@x in $.a) into 'b'

Operators (+ - * / == != < > <= >= && || ! ??) are covered in operators.

One expression per string

Each property name or value that contains an expression must hold exactly one complete expression. All of these are parse errors:

Expression	Problem
#valueOf($.a))	Leftover closing parenthesis
#valueOf($.a) @b	Two expressions with no operator between them
#valueOf($.a) ==	An operator with nothing after it

Where expressions appear

Where an expression sits decides what it can do.

Property values produce the output value for that property. Most library methods are used here.

{ "total": "#sum($.items, @x: @x.price)" }

Property names declare variables or introduce constructs that shape the output, such as #foreach, #using, #match, and #includeIf. The into keyword names the resulting property, or stores it in a variable.

{
    "@discount": "#valueOf($.discount)",
    "#foreach(@x in $.items) into 'lines'": [ { "sku": "@x.sku" } ]
}

Statements are array elements inside a #using block that modify a document in place: #setAt, #removeAt, and #when.

Match cases are property names inside a #match block: #is, #given, and #default.

Each method’s reference page lists where it’s valid under Valid in.

The arguments of a method in a property name are values, so any method you call inside them only needs to be valid in a property value:

{
    "#includeIf(#exists($.discount)) into 'discount'": {
        "amount": "#valueOf($.discount)"
    }
}

Paths and values

Most library methods accept either a JSON Path or a value. These are equivalent:

{
    "valueExists": "#exists($.some.json.path)",
    "otherValueExists": "#exists(#valueOf($.some.json.path))"
}