Question

I’m starting to play around with Node.js and specifically with the LessCSS compiler. I saw this call on the source code of the lessc script:

new(less.Parser) ({
    paths: [path.dirname(input)].concat(options.paths),
    optimization: options.optimization,
    filename: input,
    strictImports: options.strictImports,
    dumpLineNumbers: options.dumpLineNumbers
}).parse(data, function (err, tree) {
    //...
});

What I don’t understand is how that new(lessParser)({}) call works.

From what I’ve been reading about Object-oriented Javascript I only found examples of new being called like so: object = new Someting

So the questions are: how does new work in that case? Is it calling less.Parser as the constructor? Also, what are the parenthesis (following the new call) executing/returning?

Thanks!

Answer 1

The code you posted is exactly technically (or more or less) the same as doing

var parserOptions = {
    paths: [path.dirname(input)].concat(options.paths),
    optimization: options.optimization,
    filename: input,
    strictImports: options.strictImports,
    dumpLineNumbers: options.dumpLineNumbers
};
var parser = new less.Parser(parserOptions);

parser.parse(data, function (err, tree) {
    //...
});

But with a few shortcuts and with some parenthesis in “unconventional” places.

JavaScript is a loose type language and it is interpreted in tokens. Which means that it is not too limiting on it’s syntax, as long as the grammar is respected.

This let things like :

var a = false;
var b = function() { };
var c = false;
new(a || b || c)();

be completely valid, and will create a new instance of b as it is the first one non-false value defined after the tested variable a. (Note that it does not check if b is actually a function. If b was non-false and not a function, the JS engine would throw that it is not a function at run-time.)