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Embed Raw JavaScript

We're introducing this last-resort escape hatch first, in case you're ever stuck trying the other more legitimate APIs and wanna move on. Here's how you can drop a chunk of JavaScript right into your BuckleScript file:

let add = [%raw {|
  function(a, b) {
    console.log('hello from raw JavaScript!');
    return a + b;
  }
|}]

let _ = Js.log (add 1 2)

The {|foo|} syntax stands for OCaml/BuckleScript/Reason's multi-line, "quoted string" syntax. Think of them as the equivalent of JavaScript's template literals. No escaping is needed inside that string.

Careful with the OCaml/Reason syntax here. [%raw foo] allows you to embed an expression. For top-level declarations in OCaml/Reason, use [%%raw foo] (two %):

[%%raw "var a = 1"]

let f = [%raw "function() {return 1}"]

Debugger​

You can also drop a [%debugger] expression in a body:

let f x y =
  [%debugger];
  x + y

Output:

function f (x,y) {
  debugger; // JavaScript developer tools will set an breakpoint and stop here
  x + y;
}

Detect Global Variables​

BuckleScript provides a relatively type safe approach for such use case: external. [%external a_single_identifier] is a value of type option. Example:

match [%external __DEV__] with
| Some _ -> Js.log "dev mode"
| None -> Js.log "production mode"

Output:

var match = typeof (__DEV__) === "undefined" ? undefined : (__DEV__);

if (match !== undefined) {
  console.log("dev mode");
} else {
  console.log("production mode");
}

Another example:

match [%external __filename] with
| Some f -> Js.log f
| None -> Js.log "non-node environment"

Output:

var match = typeof (__filename) === "undefined" ? undefined : (__filename);

if (match !== undefined) {
  console.log(match);
} else {
  console.log("non-node environment");
}

Tips & Tricks​

Embedding raw JS snippets is discouraged, though also highly useful if you're just starting out. As a matter of fact, the first few Reason BuckleScript projects were converted through:

  • pasting raw JS snippets inside a file
  • examining the JS output (identical)
  • gradually extract a few values and functions and making sure the output still looks OK

At the end, we get a fully safe, converted Reason BuckleScript file whose JS output is clean enough that we can confidently assert that no new bug has been introduced during the conversion process.

We have a small guide on this iteration here. Feel free to peruse it later.