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February 2018


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This package provides two macros that are used to write JavaScript code inside of Julia programs.

  • The @js macro translates Julia syntax to the equivalent JavaScript
  • The @js"..." macro is used to write code using string literals with smart interpolation of values from Julia


julia> using JSExpr

julia> @js document.querySelector("#root")

julia> @js (a, b) -> a + b
JSString("(a, b) => { return a + b; }")

julia> config = Dict("foo" => "bar");
julia> js"initializeProgram($config);"


You can interpolate Julia objects or JSStrings (e.g. from other @js or js"..." invocations) as well as values from Julia (such as normal strings, Dicts, etc.).

julia> foo = 42;
julia> callback = @js a -> a + $foo
JSString("(a) => { return a + 42; }")

julia> f = @js array -> array.map($callback)
JSString("(array) => { return array.map((a) => { return a + 42; }); }")

Custom Interpolation

By default, values are serialized using the JSON package. This makes sense for Dicts, Arrays, and most other "primitive" types.

3rd-party packages can customize serialization of their own types by defining a method for JSExpr.interpolate. The return value of JSExpr.interpolate should be a JSNode.

julia> struct Link; text::String; href::String; end;
julia> JSExpr.interpolate(link::Link) = JSExpr.JSTerminal(js"<a href=$(link.href)>$(link.text)</a>");
julia> @js @const link = $(Link("Julia", "https://julialang.org/"))
JSString("const link = <a href=\"https://julialang.org/\">\"Julia\"</a>")

Object Literals

Objects are ubiquitous in JavaScript. To create objects using JSExpr, you can use a simple syntax using braces. There are two variants of this syntax (NamedTuple style and Pair style). You can also create objects use normal NamedTuple syntax.

# NamedTuple braces style
julia> @js { foo="foo", bar="bar" }
JSString("{\"foo\": \"foo\", \"bar\": \"bar\"}")

# Pair braces style (similar to Dict constructor)
julia> @js { :foo => "foo", :bar => "bar" }
JSString("{\"foo\": \"foo\", \"bar\": \"bar\"}")

# NamedTuple syntax
julia> @js (foo="foo", bar="bar")
JSString("{\"foo\": \"foo\", \"bar\": \"bar\"}")

Why not Dict?

JSExpr does not attempt to translate semantics between Julia and JavaScript (with a few very minor exceptions covered in Juliaisms below). Since Dict can be a valid function name in JavaScript, we do not translate the Julia Dict constructor to an object creation syntax.


JSExpr, for the most part, does not attempt to translate semantics between Julia and the resulting JavaScript code. The reason for the decision is that Julia and JavaScript are wildly different languages and we would invariably mess up some edge cases. We do, however, translate a few Julian constructs to a semantically equivalent JavaScript.

Range Syntax (...:...)

JavaScript doesn't have a native Range object and the typical way to repeat a loop body n times is to use a C-style for loop. There is no syntax for this style of for loop in Julia, and : is not a valid JavaScript identifier, so the colon function (:) is translated to JavaScript code that acts like a Range object in Julia.

julia> @js for i in 1:10
JSString("for (let i of (new Array(10).fill(undefined).map((_, i) => i + 1))) { console.log(i); }")

The resulting JS is very ugly and will fully materialize the range and so should only be used for relatively small ranges.


Serializing a JSString to JSON will result in a normal string containing the JavaScript code.

julia> f = @js array -> array.map($callback);
julia> JSON.print(Dict("foo" => "bar", "bar"=>f))
{"bar":"(array) => { return array.map((a) => { return a + 42; }); }","foo":"bar"}

Supported Expressions

  • Function calls
  • Comparison operators
  • Object and array literals
  • Function creation (named and anonymous functions)
  • If statements
  • For and while loops
  • JavaScript keywords (@new, @var, @let, @const)

Unsupported Expressions

Not Yet Supported

  • Ternary expressions (... ? ... : ...)
  • try / catch

Might Never Be Supported

  • Object destructuring
  • Argument splatting

If you notice anything else that's not supported or doesn't work as intended, please open an issue.

Ternary Expressions

Julia lowers (during parse) if statements and ternary expressions (... ? ... : ...) to the same Expr, so JSExpr cannot distinguish between the two. This poses an issue because JavaScript does not allow non-expression statements (e.g., loops and variable declarations) inside of a ternary expression, but if statements cannot be used in contexts which expect a value (but they can be used in such contexts in Julia).

There are plans to implement a heuristic to emit a ternary expression if appropriate (e.g., if the bodies of the ternary expression contains only one sub-expression) but this is not implemented yet.