function isJsFile(file) {
    "use strict";
    var ext = fileExt(file);
    var valid = Object.keys(require.extensions).map(function(val) {
        return val.replace(/^\./, '');
    }).filter(function(ext) {
        return ext === 'js' || ext === 'coffee';
    });
    return isString(ext, "string") && valid.indexOf(ext) !== -1;
}
Checks if a file is apparently javascript compatible (.js or .coffee).

@param  String  file  Path to the file to test
@return Boolean