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jqjs is a JavaScript implementation of the jq query language. It implements the core language features in pure JavaScript.

The main entry point to jqjs is the compile function, which turns a jq program string into a generator function:

import jq from './jq.js'
let filter = jq.compile(".x[].y")
for (let v of filter({x:[{y:2}, {y:4}]}) { ... }

The module also has a prettyPrint function for rendering an object to text.

jqjs supports most of the core jq language features, but lacks functions and some of the advanced functionality. It also uses JavaScript strings as backing, so does not have jq proper's Unicode support.

• Identity: .
• Object Identifier-Index: .foo, .foo.bar
• Generic Object Index: .[<string>]
• Array Index: .[2]
• Array/String Slice: .[10:15]
• Array/Object Value Iterator: .[]
• Comma: ,
• Pipe: |
• Parentheses: (...)
• Array Construction: [...]
• Object Construction: { ... } including shorthand { user, title } and computed keys { (.x): true }
• Recursive Descent: ..
• Addition, subtraction, multiplication, division, and modulo, including the overloaded type operations.
• Named functions
  • Built-in functions
    • tostring, tonumber, path, length, keys, has, in, type, del
    • empty, select, arrays, objects, booleans, numbers, strings, nulls
    • map, map_values, add, sort, sort_by, explode, implode, split, join
    • to_entries, from_entries, with_entries
    • range/1, range/2, range/3
    • any/0, any/1, any/2, all/0, all/1, all/2
    • contains, inside
    • the others
  • User-defined functions
  • Mathematical functions
• String interpolation: \(foo)
• Format strings and escaping: @text, @json, @html, @uri, @csv, @tsv, @sh, @base64, @base64d
  • Format interpolations: @uri "https://google.com/search?q=\(.x)"
• Equality checks: ==, !=
• Comparisons: <, >, <=, >=
  • Correct sorting order for unequal types (null < 7, [] < {})
• Conditionals: if A then B elif C then D else E end
• Alternative operator: //
• Try-catch: try EXP catch EXP
  • Error Suppression operator ?
• Regular expressions
• Variable/Symbolic Binding Operator ... as $identifier | ...
• Reduce: reduce .[] as $item (0; . + $item)
• foreach: foreach .[] as $item (...;...;...)
• Recursion: recurse(.children[])
• I/O (unlikely to make sense here)
• Update-assignment: .posts[].comments |= . + ["Another"]
• Arithmetic update-assignment: +=, -=, *=, /=, %=, //=
• Plain assignment: (.a,.b) = range(2)
• Modules with import and include

Not great.

The intention is to be semantically correct first and to have clear code second. Performance improvements sit after that, if at all. Executing a program may reëvaluate parts of it or traverse the object multiple times where that makes things simpler, and internally evaluation happens by tree-walking the input syntax.

demo.html is a live demo of how to use jqjs that lets you enter a jq program and an input JSON value and see the output JSON values it produces.