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---
language: jsonnet
filename: learnjsonnet.jsonnet
contributors:
- ["Huan Wang", "https://github.com/fredwangwang"]
---
Jsonnet is a powerful templating language for JSON. Any valid JSON
document is a valid Jsonnet object. For an interactive demo/tutorial,
click [here](https://jsonnet.org/learning/tutorial.html)
```python
// single line comment
/*
multiline comment
*/
# as well as python style comment
# define a variable.
# Variables have no effect in the generated JSON without being used.
local num1 = 1;
local num2 = 1 + 1;
local num3 = 5 - 2;
local num4 = 9 % 5;
local num5 = 10 / 2.0;
# jsonnet is a lazy language, if a variable is not used, it is not evaluated.
local num_runtime_error = 1 / 0;
# fields are valid identifiers without quotes
local obj1 = { a: 'letter a', B: 'letter B' };
local arr1 = ['a', 'b', 'c'];
# string literals use " or '.
local str1 = 'a' + 'B';
# multiline text literal in between |||
# Each line must start with a white space.
local str_multiline = |||
this is a
multiline string
|||;
# Python-compatible string formatting is available via %
# When combined with ||| this can be used for templating text files.
local str_templating = |||
%(f1)0.3f
||| % { f1: 1.2345678 };
assert str_templating == '1.235\n';
# if b then e else e. The else branch is optional and defaults to null
local var1 = if 3 < 2 then "YES";
assert var1 == null;
local obj2 = {
# variable defined inside the object ends with ','
local var_in_obj = 0,
local vowels = ['a', 'e', 'i', 'o', 'u'],
local numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
# [num] to look up an array element
first_vowel: vowels[0],
# can also slice the array like in Python
even_numbers: numbers[1::2],
# python-style list and object comprehensions are also supported
double_numbers: [x * 2 for x in numbers],
even_numbers_map: {
# [ ] syntax in field name is to compute the field name dynamically
[x + '_is_even']: true for x in numbers if x % 2 == 0
},
nested: {
nested_field1: 'some-value',
# self refers to the current object
# ["field-name"] or .field-name can be used to look up a field
nested_field2: self.nested_field1,
nested_field3: self.nested_field1,
# $ refers to outer-most object
nested_field4: $.first_vowel,
assert self.nested_field1 == self.nested_field2,
assert self.nested_field1 == self.nested_field3,
},
special_field: 'EVERYTHING FEELS BAD',
};
local obj3 = {
local var_in_obj = 1.234,
local var_in_obj2 = { a: { b: 'c' } },
concat_array: [1, 2, 3] + [4],
# strings can be concat with +,
# which implicitly converts one operand to string if needed.
concat_string: '123' + 4,
# == tests deep equality
equals: { a: { b: 'c', d: {} } } == var_in_obj2,
special_field: 'this feels good',
};
# objects can be merged with + where the right-hand side wins field conflicts
local obj4 = obj2 + obj3;
assert obj4.special_field == 'this feels good';
# define a function
# functions have positional parameters, named parameters, and default arguments
local my_function(x, y, z=1) = x + y - z;
local num6 = my_function(7, 8, 9);
local num7 = my_function(8, z=10, y=9);
local num8 = my_function(4, 5);
# inline anonymous function
local num9 = (function(x) x * x)(3);
local obj5 = {
# define a method
# fields defined with :: are hidden, which does not apper in generated JSON
# function cannot be serialized so need to be hidden
# if the object is used in the generated JSON.
is_odd(x):: x % 2 == 1,
};
assert obj5 == {};
# a jsonnet document has to evaluate to something
# be it an object, list, number or just string literal
"FIN"
```
## Further Reading
There are a few but important concepts that are not touched in this example, including:
- Passing variables from command line: [Parameterize Entire Config](https://jsonnet.org/learning/tutorial.html#parameterize-entire-config)
- Import other jsonnet libraries/files: [Imports](https://jsonnet.org/learning/tutorial.html#imports)
- In depth example of OOP aspect of Jsonnet: [Object-Orientation](https://jsonnet.org/learning/tutorial.html#Object-Orientation)
- Useful standard library: [Stdlib](https://jsonnet.org/ref/stdlib.html)
|
