JSON-e is a data-structure parameterization system for embedding context in JSON objects.

The central idea is to treat a data structure as a "template" and transform it, using another data structure as context, to produce an output data structure.

There are countless libraries to do this with strings, such as mustache. What makes JSON-e unique is that it operates on data structures, not on their textual representation. This allows input to be written in a number of formats (JSON, YAML, etc.) or even generated dynamically. It also means that the output cannot be "invalid", even when including large chunks of contextual data.

JSON-e is also designed to be safe for use on untrusted data. It never uses eval or any other function that might result in arbitrary code execution. It also disallows unbounded iteration, so any JSON-e rendering operation will finish in finite time.


See CHANGELOG.rst for the changes in each version of this library.



The JS module exposes following interface:

import jsone from 'json-e';

var template = {a: {$eval: "foo.bar"}};
var context = {foo: {bar: "zoo"}};
console.log(jsone(template, context));
// -> { a: 'zoo' }

Note that the context can contain functions, and those functions can be called from the template:

var template = {$eval: "foo(1)"};
var context = {"foo": function(x) { return x + 2; }};
console.log(jsone(template, context));  // -> 3

NOTE: Context functions are called synchronously. Any complex asynchronous operations should be handled before rendering the template.

NOTE: If the template is untrusted, it can pass arbitrary data to functions in the context, which must guard against such behavior.


The Python distribution exposes a render function:

import jsone

template = {"a": {"$eval": "foo.bar"}}
context = {"foo": {"bar": "zoo"}}
print(jsone.render(template, context))  # -> {"a": "zoo"}

and also allows custom functions in the context:

template = {"$eval": "foo(1)"}
context = {"foo": lambda x: x + 2}
print(jsone.render(template, context))  # -> 3

Go (golang)

The golang package for json-e exposes a Render function:

import (

// Template must be given using types:
//   map[string]interface{}, []interface{}, float64, string, bool, nil
// The same types that json.Unmarshal() will create when targeting an interface{}
template := map[string]interface{}{
  "result": map[string]interface{}{
    "$eval": "f() + 5",
// Context can be JSON types just like template, but may also contain functions
// these can JSON types as arguments, and return a value and optionally an error.
context := map[string]interface{}{
  "f": func() int { return 37 },

func main() {
  value, err := jsone.Render(template, context)
  fmt.Printf("%#v\n", value)

Language Reference

The examples here are given in YAML for ease of reading. Of course, the rendering operation takes place on the parsed data, so the input format is irrelevant to its operation.

Simple Operations

All JSON-e directives involve the $ character, so a template without any directives is rendered unchanged:

template: {key: [1,2,{key2: 'val', key3: 1}, true], f: false}
context:  {}
result:   {key: [1,2,{key2: 'val', key3: 1}, true], f: false}

String Interpolation

The simplest form of substitution occurs within strings, using ${..}:

template: {message: 'hello ${key}', 'k=${num}': true}
context:  {key: 'world', num: 1}
result:   {message: 'hello world', 'k=1': true}

The bit inside the ${..} is an expression, and must evaluate to something that interpolates obviously into a string (so, a string, number, boolean,). If it is null, then the expression interpolates into an empty string. The expression syntax is described in more detail below.

Values interpolate as their JSON literal values:

template: ["number: ${num}", "booleans: ${t} ${f}", "null: ${nil}"]
context: {num: 3, t: true, f: false, nil: null}
result: ["number: 3", "booleans: true false", "null: "]

Note that object keys can be interpolated, too:

template: {"tc_${name}": "${value}"}
context: {name: 'foo', value: 'bar'}
result: {"tc_foo": "bar"}

The string ${ can be escaped as $${.


JSON-e defines a bunch of operators. Each is represented as an object with a property beginning with $. This object can be buried deeply within the template. Some operators take additional arguments as properties of the same object.


The $eval operator evaluates the given expression and is replaced with the result of that evaluation. Unlike with string interpolation, the result need not be a string, but can be an arbitrary data structure.

template: {config: {$eval: 'settings.staging'}}
      transactionBackend: mock
      transactionBackend: customerdb
result:   {config: {transactionBackend: 'mock'}}

The expression syntax is described in more detail below.

Note that $eval's value must be a string. "Metaprogramming" by providing a calculated value to eval is not allowed. For example, {$eval: {$eval: "${var1} + ${var2}"}} is not valid JSON-e.


The $json operator formats the given value as JSON with sorted keys. It does not evaluate the value (use $eval for that). While this can be useful in some cases, it is an unusual case to include a JSON string in a larger data structure.

template: {$json: [a, b, {$eval: 'a+b'}, 4]}
context:  {a: 1, b: 2}
result:   '["a", "b", 3, 4]'

$if - then - else

The $if operator supports conditionals. It evaluates the given value, and replaces itself with the then or else properties. If either property is omitted, then the expression is omitted from the parent object.

template: {key: {$if: 'cond', then: 1}, k2: 3}
context:  {cond: true}
result:   {key: 1, k2: 3}
template: {$if: 'x > 5', then: 1, else: -1}
context:  {x: 10}
result:   1
template: [1, {$if: 'cond', else: 2}, 3]
context: {cond: false}
result: [1,2,3]
template: {key: {$if: 'cond', then: 2}, other: 3}
context: {cond: false}
result: {other: 3}


The $flatten operator flattens an array of arrays into one array.

template: {$flatten: [[1, 2], [3, 4], [5]]}
context:  {}
result:   [1, 2, 3, 4, 5]


The $flattenDeep operator deeply flattens an array of arrays into one array.

template: {$flattenDeep: [[1, [2, [3]]]]}
context:  {}
result:   [1, 2, 3]


The $fromNow operator is a shorthand for the built-in function fromNow. It creates a JSON (ISO 8601) datestamp for a time relative to the current time (see the now builtin, below) or, if from is given, relative to that time. The offset is specified by a sequence of number/unit pairs in a string. For example:

template: {$fromNow: '2 days 1 hour'}
context:  {}
result:   '2017-01-19T16:27:20.974Z'
template: {$fromNow: '1 hour', from: '2017-01-19T16:27:20.974Z'}
context:  {}
result:   '2017-01-19T17:27:20.974Z'

The available units are day, hour, and minute, for all of which a plural is also accepted.


The $let operator evaluates an expression using a context amended with the given values. It is analogous to the Haskell where clause.

template: {$let: {ts: 100, foo: 200},
           in: [{$eval: "ts+foo"}, {$eval: "ts-foo"}, {$eval: "ts*foo"}]}
context: {}
result: [300, -100, 20000]

The $let operator here added the ts and foo variables to the scope of the context and accordingly evaluated the in clause using those variables to return the correct result.

The variable names in the $let value must be valid context variable names and must be written literally. That is, an expression like {$let: {$eval: "extraVariables"}, in : ..} is not allowed.


The $map operator evaluates an expression for each value of the given array or object, constructing the result as an array or object of the evaluated values.

When given an array, map always returns an array.

  $map: [2, 4, 6]
  each(x): {$eval: 'x + a'}
context:  {a: 1}
result:   [3, 5, 7]

The array or object is the value of the $map property, and the expression to evaluate is given by each(var) where var is the name of the variable containing each element. In the case of iterating over an object, var will be an object with two keys: key and val. These keys correspond to a key in the object and its corresponding value.

When $map is given an object, the expression defined by each(var) must evaluate to an object for each key/value pair (key and val).The objects constructed by each 'each(var)' can then be merged internally to give the resulting object with later keys overwriting the previous ones.Otherwise the expression becomes invalid for the $map operator

  $map: {a: 1, b: 2, c: 3}
  each(y): {'${y.key}x': {$eval: 'y.val + 1'}}
context:  {}
result: {ax: 2, bx: 3, cx: 4}


The $merge operator merges an array of objects, returning a single object that combines all of the objects in the array, where the right-side objects overwrite the values of the left-side ones.

template: {$merge: [{a: 1, b: 1}, {b: 2, c: 3}, {d: 4}]}
context:  {}
result:   {a: 1, b: 2, c: 3, d: 4}


The $mergeDeep operator is like $merge, but it recurses into objects to combine their contents property by property. Arrays are concatenated.

    - task:
          command: [a, b]
    - task:
          foo: bar
    - task:
          command: [c]
context:  {}
      foo: bar
      command: [a, b, c]


The $sort operator sorts the given array. It takes a by(var) property which should evaluate to a comparable value for each element. The by(var) property defaults to the identity function.

  $sort: [{a: 2}, {a: 1, b: []}, {a: 3}]
  by(x): 'x.a'
context:  {}
result:   [{a: 1, b: []}, {a: 2}, {a: 3}]


The $reverse operator simply reverses the given array.

template: {$reverse: [3, 4, 1, 2]}
context:  {}
result:   [2, 1, 4, 3]

Escaping operators

All property names starting with $ are reserved for JSON-e. You can use $$ to escape such properties:

template: {$$reverse: [3, 2, {$$eval: '2 - 1'}, 0]}
context:  {}
result:   {$reverse: [3, 2, {$eval: '2 - 1'}, 0]}


Many values can be evaluated in context where booleans are required, not just booleans themselves. JSON-e defines the following values as false. Anything else will be true.

template: {$if: 'a || b || c || d || e || f', then: "uh oh", else: "falsy" }
context: {a: null, b: [], c: {}, d: "", e: 0, f: false}
result: "falsy"

Expression Syntax

Expression are given in a simple Python- or JavaScript-like expression language. Its data types are limited to JSON types plus function objects.


Literals are similar to those for JSON. Numeric literals only accept integer and decimal notation. Strings do not support any kind of escaping. The use of \n and \t in the example below depends on the YAML parser to expand the escapes.

  - {$eval: "1.3"}
  - {$eval: "'abc'"}
  - {$eval: '"abc"'}
  - {$eval: "'\n\t'"}
context: {}
  - 1.3
  - "abc"
  - "abc"
  - "\n\t"

Array and object literals also look much like JSON, with bare identifiers allowed as keys like in Javascript:

  - {$eval: '[1, 2, "three"]'}
  - {$eval: '{foo: 1, "bar": 2}'}
context: {}
  - [1, 2, "three"]
  - {"foo": 1, "bar": 2}

Context References

Bare identifiers refer to items from the context or to built-ins (described below).

template: {$eval: '[x, z, x+z]'}
context: {x: 'quick', z: 'sort'}
reslut: ['quick', 'sort', 'quicksort']

Arithmetic Operations

The usual arithmetic operators are all defined, with typical associativity and precedence:

  - {$eval: 'x + z'}
  - {$eval: 's + t'}
  - {$eval: 'z - x'}
  - {$eval: 'x * z'}
  - {$eval: 'z / x'}
  - {$eval: 'z ** 2'}
  - {$eval: '(z / x) ** 2'}
context: {x: 10, z: 20, s: "face", t: "plant"}
  - 30
  - "faceplant"
  - 10
  - 200
  - 2
  - 400
  - 4

Note that strings can be concatenated with +, but none of the other operators apply.

Comparison Operations

Comparisons work as expected. Equality is "deep" in the sense of doing comparisons of the contents of data structures.

  - {$eval: 'x < z'}
  - {$eval: 'x <= z'}
  - {$eval: 'x > z'}
  - {$eval: 'x >= z'}
  - {$eval: 'deep == [1, [3, {a: 5}]]'}
  - {$eval: 'deep != [1, [3, {a: 5}]]'}
context: {x: -10, z: 10, deep: [1, [3, {a: 5}]]}
result: [true, true, false, false, true, false]

Boolean Operations

Boolean operations use C- and Javascript-style symbls ||, &&, and !:

template: {$eval: '!(false || false) && true'}
context: {}
result: true

Object Property Access

Like Javascript, object properties can be accessed either with array-index syntax or with dot syntax. Unlike Javascript, obj.prop is an error if obj does not have prop, while obj['prop'] will evaulate to null.

template: {$eval: 'v.a + v["b"]'}
context: {v: {a: 'apple', b: 'bananna', c: 'carrot'}}
result: 'applebananna'

Indexing and Slicing

Strings and arrays can be indexed and sliced using a Python-like indexing scheme. Negative indexes are counted from the end of the value. Slices are treated as "half-open", meaning that the result contains the first index and does not contain the second index. A "backward" slice with the start index greater than the end index is treated as empty.

  - {$eval: '[array[1], string[1]]'}
  - {$eval: '[array[1:4], string[1:4]]'}
  - {$eval: '[array[2:], string[2:]]'}
  - {$eval: '[array[:2], string[:2]]'}
  - {$eval: '[array[4:2], string[4:2]]'}
  - {$eval: '[array[-2], string[-2]]'}
  - {$eval: '[array[-2:], string[-2:]]'}
  - {$eval: '[array[:-3], string[:-3]]'}
context: {array: ['a', 'b', 'c', 'd', 'e'], string: 'abcde'}
  - ['b', 'b']
  - [['b', 'c', 'd'], 'bcd']
  - [['c', 'd', 'e'], 'cde']
  - [['a', 'b'], 'ab']
  - [[], '']
  - ['d', 'd']
  - [['d', 'e'], 'de']
  - [['a', 'b'], 'ab']

Containment Operation

The in keyword can be used to check for containment: a property in an object, an element in an array, or a substring in a string.

  - {$eval: '"foo" in {foo: 1, bar: 2}'}
  - {$eval: '"foo" in ["foo", "bar"]'}
  - {$eval: '"foo" in "foobar"'}
context: {}
result: [true, true, true]

Function Invocation

Function calls are made with the usual fn(arg1, arg2) syntax. Functions are not JSON data, so they cannot be created in JSON-e, but they can be provided as built-ins or supplied in the context and called from JSON-e.

Built-In Functions and Variables

The expression language provides a laundry-list of built-in functions/variables. Library users can easily add additional functions/variables, or override the built-ins, as part of the context.


The built-in context value now is set to the current time at the start of evaluation of the template, and used as the default "from" value for $fromNow and the built-in fromNow().

  - {$eval: 'now'}
  - {$eval: 'fromNow("1 minute")'}
  - {$eval: 'fromNow("1 minute", "2017-01-19T16:27:20.974Z")'}
context: {}
  - '2017-01-19T16:27:20.974Z',
  - '2017-01-19T16:28:20.974Z',
  - '2017-01-19T16:28:20.974Z',


  # the smallest of the arguments
  - {$eval: 'min(1, 3, 5)'}
  # the largest of the arguments
  - {$eval: 'max(2, 4, 6)'}
  # mathematical functions
  - {$eval: 'sqrt(16)'}
  - {$eval: 'ceil(0.3)'}
  - {$eval: 'floor(0.3)'}
  - {$eval: 'abs(-0.3)'}
context: {}
  - 1
  - 6
  - 4
  - 1
  - 0
  - 0.3


  # convert string case
  - {$eval: 'lowercase("Fools!")'}
  - {$eval: 'uppercase("Fools!")'}
  # convert string, number, boolean, or array to string
  - {$eval: 'str(130)'}
  # strip whitespace from left, right, or both ends of a string
  - {$eval: 'lstrip("  room  ")'}
  - {$eval: 'rstrip("  room  ")'}
  - {$eval: 'strip("  room  ")'}
context: {}
  - "fools!"
  - "FOOLS!"
  - "130"
  - "room  "
  - "  room"
  - room


The len() built-in returns the length of a string or array.

template: {$eval: 'len([1, 2, 3])'}
context: {}
result: 3