jsq

.

The absolute simplest (and least interesting) filter is .. This is a filter that takes its input and produces it unchanged as output.

.foo

The simplest useful filter is .foo. When given a JSON object as input, it produces the value at the key "foo", or nothing if there's none present.

.["foo"]

You can also look up fields of an object using syntax like .["foo"] (.foo above is a shorthand version of this). This one works for arrays as well, if the key is an integer.

.[]

If you use the .["foo"] syntax, but omit the index entirely, it will return all of the elements of an array. Running .[] with the input [1,2,3] will produce the numbers as a list of three results, rather than as a single array. This also works for objects.

,

If two filters are separated by a comma, then the input will be fed into both and there will be multiple outputs: first, all of the outputs produced by the left expression, and then all of the outputs produced by the right. For instance, filter .foo, .bar, produces both the "foo" fields and "bar" fields as separate outputs.

By combining expression like this, you are creating what is called a list. You can read more about this in the next chapter Types and Values.

|

The | operator combines two filters by feeding the result(s) of the one on the left into the input of the one on the right. It's pretty much the same as the Unix shell's pipe, if you're used to that.

If the one on the left produces multiple results, the one on the right will be run for each of those results. So, the expression .[] | .foo retrieves the "foo" field of each result of the input expression.

Simple values

Strings, numbers, booleans and null are written the same way as in javascript. Just like everything else in jsq, these simple values take an input and produce an output — 42 is a valid jsq expression that takes an input, ignores it, and returns 42 instead.

Like in JSON, strings are delimited using "double quotes". 'Single quotes' are not allowed.

Arrays — []

Similar to JavaScript, [] is used to construct arrays, as in [1,2,3]. The elements of the array can be any jsq expression. All of the results produced by all of the expressions are collected into one big array. You can use it to construct an array out of a known quantity of values (as in [.foo, .bar, .baz]) or to "collect" all the results of a filter into an array (as in [.items[].name])

If you have a filter X that produces four results, then the expression [X] will produce a single result, an array of four elements.

Lists — 1,2,3

Once you understand the , operator, you can look at jsq's array syntax in a different light: the expression [1,2,3] is not using a built-in syntax for comma-separated arrays, but is actually applying the [] operator (collect results) to the expression 1,2,3 which produces a list of 3 integers.

A list will concatenate all results from all elements of the list.

Objects — {}

Continuing JavaScript's familiarity, objects are constructed using {} — e.g.: {"a": 42, "b": 17}.

If the keys are "sensible" (all alphanumeric characters), then the quotes can be left off. The value can be any expression (although you may need to wrap it in parentheses if it's a complicated one — like when using a list), which gets applied to the {} expression's input (remember, all filters have an input and an output).

{foo: .bar}

will produce the JSON object {"foo": 42} if given the JSON object {"bar":42, "baz":43}. You can use this to select particular fields of an object: if the input is an object with "user", "title", "id", and "content" fields and you just want "user" and "title", you can write

{user: .user, title: .title}

Because that's so common, there's a shortcut syntax: {user, title}.

If one of the expressions produces multiple results, multiple dictionaries will be produced. If the input's

{
  "user":"tjoekbezoer",
  "titles":["JSQ Primer", "More JSQ"]
}

then the expression

{user, title: .titles[]}

will produce two outputs:

[
  {"user":"tjoekbezoer", "title": "JSQ Primer"},
  {"user":"tjoekbezoer", "title": "More JSQ"}
]

Putting parentheses around the key means it will be evaluated as an expression. With the same input as above,

{(.user): .titles}

produces

{"tjoekbezoer": ["JSQ Primer", "More JSQ"]}

Simple expressions, like a single filter, can also be used without the parentheses. A filter used as a key name can return only one result, or jsq will throw an error.

Addition - +

The operator + takes two filters, applies them both to the same input, and adds the results together. What "adding" means depends on the types involved:

• Numbers are added by normal arithmetic.

• Arrays are added by being concatenated into a larger array.

• Strings are added by being joined into a larger string.

• Objects are added by merging, that is, inserting all the key-value pairs from both objects into a single combined object. If both objects contain a value for the same key, the object on the right of the + will be used.

Subtraction - -

• Numbers are subtracted by normal arithmetic.
• Arrays are being subtracted by removing all occurences of the second array's elements from the first array.
• Objects are subtracted similarly — all keys from the second object will be removed from the first object.

Multiplication, division - * and /

These operators only work on numbers, and do the expected.

Binary — and, or and xor

These operators are similar to JavaScript's &, | and ^.

=

The assignment operation .foo = 1 will take as input an object and produce as output an object with the "foo" field set to 1. There is no notion of "modifying" or "changing" something in jsq - all jsq values are immutable. For instance,

.foo = .bar | .foo.baz = 1

will not have the side-effect of setting .bar.baz to be set to 1, as the similar-looking program in Javascript, Python, Ruby or other languages would. Unlike these languages, there is no notion of two arrays or objects being "the same array" or "the same object". They can be equal, or not equal, but if we change one of them in no circumstances will the other change behind our backs.

This means that it's impossible to build circular values in jsq (such as an array whose first element is itself). This is quite intentional, and ensures that anything a jsq program can produce can be represented in JSON.

|=

As well as the assignment operator '=', jsq provides the "update" operator '|=', which takes a filter on the right-hand side and works out the new value for the property being assigned to by running the old value through this expression. For instance, .foo |= .+1 will build an object with the "foo" field set to the input's "foo" plus 1.

This example should show the difference between '=' and '|='. Provide input

{"a": {"b": 10}, "b": 20}

to .a = .b and it produces {"a": 20, "b": 20}.
to .a |= .b and it produces {"a": 10, "b": 20}.

+=, -=, *=, /=

jsq has a few operators of the form a op= b, which are all equivalent to a |= . op b. So for example, += 1 can be used to increment values.

Complex assignments

Lots more things are allowed on the left-hand side of a jsq assignment than in most langauges. We've already seen simple field accesses on the left hand side, and it's no surprise that array accesses work just as well:

.posts[0].title = "JSQ Manual"

What may come as a surprise is that the expression on the left may produce multiple results, referring to different points in the input document:

.posts[].comments |= . + ["this is great"]

That example appends the string "this is great" to the "comments" array of each post in the input (where the input is an object with a field "posts" which is an array of posts).

When jsq encounters an assignment like 'a = b', it records the "path" taken to select a part of the input document while executing a. This path is then used to find which part of the input to change while executing the assignment. Any filter may be used on the left-hand side of an equals - whichever paths it selects from the input will be where the assignment is performed.

==, !=, ===

For scalars, these comparison operators behave the same as their Javascript equivalents. For arrays and objects, a deep comparison is performed.

>, >=, <=, <

The comparison operators >, >=, <=, < behave as expected. They can also be used to compare arrays and objects.

The ordering is the same as that described for the built-in function sort, below.

if(cond, then[, else])

Consider the expression if(A, B, C). If A evaluates as truthy, B will be executed. Otherwise it will execute C, if provided.

If the condition A is an expression producing multiple results, it is considered "true" if any of those results is truthy. If it produces zero results, it's considered false.

More about truthy values: Truth, equality and JavaScript

&&, ||, !

jsq supports the normal Boolean operators && and ||. They have the same standard of truth as if expressions.

If an operand of one of these operators produces multiple results, the operator itself will produce a result for each input.

The boolean operators behave the same as JavaScript's, returning the value of the last correct operand. So for example 0 && 2 returns 2; !1 || 3 returns 3.

! simply negates the proceding value.

length

The built-in function length gets the length of various different types of values:

• The length of a string is the number of characters.
• The length of an array is the number of elements.
• The length of an object is the number of key-value pairs.
• The length of null is zero.

When trying to get the length of a number, it will return nothing.

keys

The built-in function keys, when given an object, returns its keys in a list. When given an array, it returns the valid indices for that array: the integers from 0 to length-1.

When 'keying' an object, do not rely on the sort order. The order in which the keys are returned is defined by the browser implementation of object iteration, which is not cross-browser consistent.

pairs

When passed an object, pairs will return a list of arrays, where every array is a key-value combination. This also works on arrays.

Like with the keys function, do not rely on sort order when 'pairing' an object.

select

The function select(foo) produces its input unchanged if foo returns true for that input, and produces no output otherwise.

It's useful for filtering lists, objects and arrays: [1,2,3] | map(select(. >= 2)) will give you [2,3].

empty

empty returns no results. None at all. Not even null.

map(x)

For any filter x, map(x) will run that filter for each element of the input object/array, and produce a list of results equal to the amount of elements. map(.+1) will increment each element of an array of numbers.

map(x) is equivalent to .[] | x.

add

The filter add takes as input an array, and produces as output the elements of the array added together. This might mean summed, concatenated or merged depending on the types of the elements of the input array - the rules are the same as those for the + operator (described above).

If the input is an empty array, add returns null.

tonumber

The tonumber function will parse its input as a number. It's not able to parse objects and arrays, nor can it parse strings not starting with a number. In these cases, tonumber will return null. Parsing booleans results in 1 or 0.

tostring

The tostring function prints its input as a string. Jsq uses JSON.stringify for this, so make sure it is available (in older browser you might need to include json2.js)

sort

The sort function sorts its input, which must be an array. Values are sorted in the following order:

• null
• false
• true
• numbers
• strings, in alphabetical order (by unicode codepoint value)
• arrays, in lexical order
• objects

The ordering for objects is a little complex: first they're compared by comparing their sets of keys (as arrays in sorted order), and if their keys are equal then the values are compared key by key.

min, max

Find the minimum or maximum element of the input array. Providing a filter as argument allows you to specify a particular field or property to examine, e.g. min(.foo) finds the object with the smallest foo field.

not

This function simply negates the input it receives.

recurse

The recurse function allows you to search through a recursive structure, and extract interesting data from all levels. Suppose your input represents a filesystem:

{"name": "/", "children": [
  {"name": "/bin", "children": [
    {"name": "/bin/ls", "children": []},
    {"name": "/bin/sh", "children": []}
  ]},
  {"name": "/home", "children": [
    {"name": "/home/daan", "children": [
      {"name": "/home/daan/jsq", "children": []}
    ]}
  ]}
]}

Now suppose you want to extract all of the filenames present. You need to retrieve .name, .children[].name, .children[].children[].name, and so on. You can do this with:

recurse(.children[]) | .name

format(string)

Much like a simplified sprintf, format accepts an array of values to be integrated into a custom string.