Parse javascript-like object literals in Julia into a Julia object
Author davidavdav
5 Stars
Updated Last
2 Years Ago
Started In
February 2019


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Parse javascript-like object literals in Julia into a Julia object

This package provides a macro @js that parses a Julia expression interpreted as javascript, and tries to form a Julia object from that. You can use javascript shortcuts like @js { a: b } to write Dict("a" => b) and even @js { a } to write Dict("a" => a).

I am not sure if this is useful for anything else than for me trying to understand macros and evaluation in Julia...

@js expression

Parses the Julia expression as a javascript object iteral. It can be called either as @js expression or @js(expression), the latter case explicitly delimits the extent of the expression being parsed by the macro.


## input
e = 5.0
g = "gee!"
@js {
  a: 1,
  b: [2, 3 * 3, ],
  c : {
    d: "doubly-quoted string",
  f.g: g
## results
Dict{String,Any} with 4 entries:
  "f" => Dict{String,Any}("g"=>"gee!")
  "c" => Dict{String,Any}("e"=>5.0,"d"=>"doubly-quoted string")
  "b" => Any[2, 9, sqrt]
  "a" => 1

All dicts created in the process are always of type Dict{String,Any}, and all arrays are of type Array{Any}, to cater for future assignments of the elements to different types.

Please note that we can't fully parse all javascript object literals, as Julia can't interpret singly-quoted strings as strings, only single characters can be parsed like this.

Deep object traversal with @js

@js(dotted expression) traverses a hierachical json-like object directly.


a = @js { b: { c: { d: 4 } } }
@js(a.b.c.d) == 4
@js(a.b.c) == Dict("d" => 4)
@js(a.b) == @js { c: { d: 4 } }
@js(a.b) == @js { c.d: 4 }

The RHS in the last expression shows object creation in deep traversal. Standard javascript does not allow this.

You can mix strings as keys with indices.

d = @js { a: 1, b: [1, {c: 2}, 3], d: 4}
@js d.b[2].c


You make assignments in the @js expression:

@js a = { b: 3 }
@js a.b = 4
@js a.b = { c: 5 }
@js a.b.c = 6
@js a, b = [ { c: 3}, { d: 4} ]
@js dict = { d: π, e: [ 1, { f: 2}, 3], c: sin }
@js { c, d, e } = dict
c == sin ## true
d == π ## true
e == [ 1, Dict("f" => 2), 3] ## true

The JSObject struct

We also have some support for dot-notation of JS-like objects in a native Julia struct. This might already have been implemented before, and probably better, so please submit an issue "irrelevant code" if you know of any better implementation.

The JSObject struct can wrap a classic Dict/Array based JS-like object, thus providing native Julia support for dot notation, using the getproperty() function.


a = JSObject(@js({ b: { c: { d: 4 } } }))
a.b.c.d == 4 ## true
a["b"].c["d"] == 4 ## true

a.c = @js {d: { e: [5, 6] } }
a.c.d.e[2] == 6 ## true

b = JSObject(@js([{c: 3}, {d: [4, {e: 5}]}]))
b[1].c == 3
b[2].d[1] == 4
b[2].d[2].e == 5

The JSObject can hold a Dict{String, Any} or a Vector{Any} (or even a plain number or String, but that is not so useful).

In assigning to a member of a JSObject, as in a.c = @js {d: { e: [5, 6]}}, the RHS is automatically wrapped in a JSObject for consistency.

The constructors JSObject() parse all the values, and recursively make them into JSObjects in case they are of type Dict or Vector. Other types are left as-is, so deeper Ints or floats are not unnecissarily wrapped.

Exporting to a plain old Julias JSON structure

You can access members of the JSObject struct using indexing [] and . notation. But you may need to export the JSObject as a plain old JSON, consisting of ordinary Dicts and Arrays. You can do that using


In fact, this happens in the show(io::IO, object::JSObject) function to consicely display a variable of type JSObject, using JSON.json().