--- title: "Formatting code with PhantomJS and CodeMirror" author: "St├ęphane Laurent" date: '2020-10-21' tags: javascript output: md_document: variant: markdown preserve_yaml: true html_document: highlight: tango theme: cosmo highlighter: pandoc-solarized --- Here is some ugly Julia code: ```julia function mandelbrot(a) z = 0 for i=1:50 z = z^2 + a end return z end for y=1.0:-0.05:-1.0 for x=-2.0:0.0315:0.5 abs(mandelbrot(complex(x, y))) < 2 ? print("*") : print(" ") end println() end ``` We will see how to format it with the help of [CodeMirror](https://codemirror.net/) and [PhantomJS](https://phantomjs.org/). The result we will get is: ```julia function mandelbrot(a) z = 0 for i=1:50 z = z^2 + a end return z end for y=1.0:-0.05:-1.0 for x=-2.0:0.0315:0.5 abs(mandelbrot(complex(x, y))) < 2 ? print("*") : print(" ") end println() end ``` If necessary, install `PhantomJS`. If you are a R user, you can install it with the help of the 'webdriver' package, by running `webdriver::install_phantomjs()`. Now, download these files (these are `CodeMirror` components): - [codemirror.js](http://codemirror.net/lib/codemirror.js) - [julia.js](http://codemirror.net/mode/julia/julia.js) - [formatting.js](http://codemirror.net/2/lib/util/formatting.js) Below is the `PhantomJS` script. Save it into a file named **JuliaFormatter.js**. ```{js, attr.source='.numberLines'} "use strict"; phantom.onError = function (msg, trace) { console.log(msg); phantom.exit(1); }; // read the arguments of the call var system = require("system"); var args = system.args; // = [script, julia file, indentation size] // open the Julia file and put its contents in the 'code' variable var fs = require("fs"); var code = fs.read(args[1]); // create a page var page = require("webpage").create(); page.onConsoleMessage = function (str) { console.log(str); }; // main stuff page.open("about:blank", function (status) { if (status === "success") { page.content = '"; if (page.injectJs("codemirror.js")) { if (page.injectJs("julia.js")) { if (page.injectJs("formatting.js")) { page.evaluate(function (tabSize) { var editor = CodeMirror.fromTextArea( document.getElementById("editor"), { mode: "julia", tabSize: tabSize, indentUnit: tabSize } ); CodeMirror.commands.selectAll(editor); editor.autoFormatRange( editor.getCursor(true), editor.getCursor(false) ); editor.setCursor(0); console.log(editor.getValue()); }, parseInt(args[2])); } else { console.error("injectJs failed"); phantom.exit(1); } } else { console.error("injectJs failed"); phantom.exit(1); } } else { console.error("injectJs failed"); phantom.exit(1); } } else { console.error("page.open failed"); phantom.exit(1); } phantom.exit(0); }); ``` The script takes two arguments: the file containing the Julia code, say **mandelbrot.jl**, and an integer, the desired number of spaces of the indentation. Below is the command to execute it, and the output: ``` $ phantomjs JuliaFormatter.js mandelbrot.jl 2 function mandelbrot(a) z = 0 for i=1:50 z = z^2 + a end return z end for y=1.0:-0.05:-1.0 for x=-2.0:0.0315:0.5 abs(mandelbrot(complex(x, y))) < 2 ? print("*") : print(" ") end println() end ``` `PhantomJS` is a [headless browser](https://en.wikipedia.org/wiki/Headless_browser): > A headless browser is a web browser without a graphical user interface. Headless browsers provide automated control of a web page in an environment similar to popular web browsers, but they are executed via a command-line interface or using network communication. They are particularly useful for testing web pages as they are able to render and **understand HTML the same way a browser would**, including styling elements such as page layout, colour, font selection and execution of JavaScript and Ajax which are usually not available when using other testing methods. So you can create a HTML element with `PhantomJS`, such as a `textarea`, and you can get it with `document.getElementById`, exactly as you would do with a normal browser. What does the script do: - it creates a blank HTML page - it fills it with a `textarea` displaying the code contained in the Julia file (but nothing is displayed to the user!) - it runs JavaScript code using `CodeMirror` to format the code. `CodeMirror` is a client-side library. It does not allow to format the code on the JavaScript side exclusively: this code must be rendered in the browser. This is why one needs a headless browser to use `CodeMirror` without using a normal browser. My R package [prettifyAddins](https://github.com/stla/prettifyAddins) uses this method to format C, C++, Java, Fortran, Julia, Python, SAS, Scala, Shell, and SQL. There are many other languages supported by `CodeMirror`.