node_modules/browser-resolve/index.js

// builtin
var fs = require('fs');
var path = require('path');

// vendor
var resv = require('resolve');

// given a path, create an array of node_module paths for it
// borrowed from substack/resolve
function nodeModulesPaths (start, cb) {
    var splitRe = process.platform === 'win32' ? /[\/\\]/ : /\/+/;
    var parts = start.split(splitRe);

    var dirs = [];
    for (var i = parts.length - 1; i >= 0; i--) {
        if (parts[i] === 'node_modules') continue;
        var dir = path.join.apply(
            path, parts.slice(0, i + 1).concat(['node_modules'])
        );
        if (!parts[0].match(/([A-Za-z]:)/)) {
            dir = '/' + dir;
        }
        dirs.push(dir);
    }
    return dirs;
}

function find_shims_in_package(pkgJson, cur_path, shims, browser) {
    try {
        var info = JSON.parse(pkgJson);
    }
    catch (err) {
        err.message = pkgJson + ' : ' + err.message
        throw err;
    }

    var replacements = getReplacements(info, browser);

    // no replacements, skip shims
    if (!replacements) {
        return;
    }

    // if browser mapping is a string
    // then it just replaces the main entry point
    if (typeof replacements === 'string') {
        var key = path.resolve(cur_path, info.main || 'index.js');
        shims[key] = path.resolve(cur_path, replacements);
        return;
    }

    // http://nodejs.org/api/modules.html#modules_loading_from_node_modules_folders
    Object.keys(replacements).forEach(function(key) {
        var val;
        if (replacements[key] === false) {
            val = path.normalize(__dirname + '/empty.js');
        }
        else {
            val = replacements[key];
            // if target is a relative path, then resolve
            // otherwise we assume target is a module
            if (val[0] === '.') {
                val = path.resolve(cur_path, val);
            }
        }

        if (key[0] === '/' || key[0] === '.') {
            // if begins with / ../ or ./ then we must resolve to a full path
            key = path.resolve(cur_path, key);
        }
        shims[key] = val;
    });

    [ '.js', '.json' ].forEach(function (ext) {
        Object.keys(shims).forEach(function (key) {
            if (!shims[key + ext]) {
                shims[key + ext] = shims[key];
            }
        });
    });
}

// paths is mutated
// load shims from first package.json file found
function load_shims(paths, browser, cb) {
    // identify if our file should be replaced per the browser field
    // original filename|id -> replacement
    var shims = Object.create(null);

    (function next() {
        var cur_path = paths.shift();
        if (!cur_path) {
            return cb(null, shims);
        }

        var pkg_path = path.join(cur_path, 'package.json');

        fs.readFile(pkg_path, 'utf8', function(err, data) {
            if (err) {
                // ignore paths we can't open
                // avoids an exists check
                if (err.code === 'ENOENT') {
                    return next();
                }

                return cb(err);
            }
            try {
                find_shims_in_package(data, cur_path, shims, browser);
                return cb(null, shims);
            }
            catch (err) {
                return cb(err);
            }
        });
    })();
};

// paths is mutated
// synchronously load shims from first package.json file found
function load_shims_sync(paths, browser) {
    // identify if our file should be replaced per the browser field
    // original filename|id -> replacement
    var shims = Object.create(null);
    var cur_path;

    while (cur_path = paths.shift()) {
        var pkg_path = path.join(cur_path, 'package.json');

        try {
            var data = fs.readFileSync(pkg_path, 'utf8');
            find_shims_in_package(data, cur_path, shims, browser);
            return shims;
        }
        catch (err) {
            // ignore paths we can't open
            // avoids an exists check
            if (err.code === 'ENOENT') {
                continue;
            }

            throw err;
        }
    }
    return shims;
}

function build_resolve_opts(opts, base) {
    var packageFilter = opts.packageFilter;
    var browser = normalizeBrowserFieldName(opts.browser)

    opts.basedir = base;
    opts.packageFilter = function (info, pkgdir) {
        if (packageFilter) info = packageFilter(info, pkgdir);

        var replacements = getReplacements(info, browser);

        // no browser field, keep info unchanged
        if (!replacements) {
            return info;
        }

        info[browser] = replacements;

        // replace main
        if (typeof replacements === 'string') {
            info.main = replacements;
            return info;
        }

        var replace_main = replacements[info.main || './index.js'] ||
            replacements['./' + info.main || './index.js'];

        info.main = replace_main || info.main;
        return info;
    };

    var pathFilter = opts.pathFilter;
    opts.pathFilter = function(info, resvPath, relativePath) {
        if (relativePath[0] != '.') {
            relativePath = './' + relativePath;
        }
        var mappedPath;
        if (pathFilter) {
            mappedPath = pathFilter.apply(this, arguments);
        }
        if (mappedPath) {
            return mappedPath;
        }

        var replacements = info[browser];
        if (!replacements) {
            return;
        }

        mappedPath = replacements[relativePath];
        if (!mappedPath && path.extname(relativePath) === '') {
            mappedPath = replacements[relativePath + '.js'];
            if (!mappedPath) {
                mappedPath = replacements[relativePath + '.json'];
            }
        }
        return mappedPath;
    };

    return opts;
}

function resolve(id, opts, cb) {

    // opts.filename
    // opts.paths
    // opts.modules
    // opts.packageFilter

    opts = opts || {};
    opts.filename = opts.filename || '';

    var base = path.dirname(opts.filename);

    if (opts.basedir) {
        base = opts.basedir;
    }

    var paths = nodeModulesPaths(base);

    if (opts.paths) {
        paths.push.apply(paths, opts.paths);
    }

    paths = paths.map(function(p) {
        return path.dirname(p);
    });

    // we must always load shims because the browser field could shim out a module
    load_shims(paths, opts.browser, function(err, shims) {
        if (err) {
            return cb(err);
        }

        var resid = path.resolve(opts.basedir || path.dirname(opts.filename), id);
        if (shims[id] || shims[resid]) {
            var xid = shims[id] ? id : resid;
            // if the shim was is an absolute path, it was fully resolved
            if (shims[xid][0] === '/') {
                return resv(shims[xid], build_resolve_opts(opts, base), function(err, full, pkg) {
                    cb(null, full, pkg);
                });
            }

            // module -> alt-module shims
            id = shims[xid];
        }

        var modules = opts.modules || Object.create(null);
        var shim_path = modules[id];
        if (shim_path) {
            return cb(null, shim_path);
        }

        // our browser field resolver
        // if browser field is an object tho?
        var full = resv(id, build_resolve_opts(opts, base), function(err, full, pkg) {
            if (err) {
                return cb(err);
            }

            var resolved = (shims) ? shims[full] || full : full;
            cb(null, resolved, pkg);
        });
    });
};

resolve.sync = function (id, opts) {

    // opts.filename
    // opts.paths
    // opts.modules
    // opts.packageFilter

    opts = opts || {};
    opts.filename = opts.filename || '';

    var base = path.dirname(opts.filename);

    if (opts.basedir) {
        base = opts.basedir;
    }

    var paths = nodeModulesPaths(base);

    if (opts.paths) {
        paths.push.apply(paths, opts.paths);
    }

    paths = paths.map(function(p) {
        return path.dirname(p);
    });

    // we must always load shims because the browser field could shim out a module
    var shims = load_shims_sync(paths, opts.browser);
    var resid = path.resolve(opts.basedir || path.dirname(opts.filename), id);

    if (shims[id] || shims[resid]) {
        var xid = shims[id] ? id : resid;
        // if the shim was is an absolute path, it was fully resolved
        if (shims[xid][0] === '/') {
            return resv.sync(shims[xid], build_resolve_opts(opts, base));
        }

        // module -> alt-module shims
        id = shims[xid];
    }

    var modules = opts.modules || Object.create(null);
    var shim_path = modules[id];
    if (shim_path) {
        return shim_path;
    }

    // our browser field resolver
    // if browser field is an object tho?
    var full = resv.sync(id, build_resolve_opts(opts, base));

    return (shims) ? shims[full] || full : full;
};

function normalizeBrowserFieldName(browser) {
    return browser || 'browser';
}

function getReplacements(info, browser) {
    browser = normalizeBrowserFieldName(browser);
    var replacements = info[browser] || info.browser;

    // support legacy browserify field for easier migration from legacy
    // many packages used this field historically
    if (typeof info.browserify === 'string' && !replacements) {
        replacements = info.browserify;
    }

    return replacements;
}

module.exports = resolve;