Catching module loading errors and processing them

It is also possible to use errbacks to have customized error handling appropriate to the specific use of require. Errbacks are documented here http://requirejs.org/docs/api.html#errbacks. Basically, you can add to require a function to be called if the load fails. It comes right after the function to be called if the load is successful.

Chin’s case could be handled as:

require([path], function(content){
  //need to catch errors as this will not be called;
}, function (err) {
  //display error to user
});

Here’s an example that tries loading from multiple places:

require([mode_path], onload, function (err) {

    if (mode_path.indexOf("https://stackoverflow.com/") !== -1)
        // It is an actual path so don't try any further loading
        throw new Error("can't load mode " + mode_path);

    var path = "./modes/" + mode_path + "https://stackoverflow.com/" + mode_path;
    require([path], onload,
            function (err) {
        require([path + "_mode"], onload);
    });
});

In this example onload would be the function called once the required code loads, and mode_path is a string identifying the mode. What you see there is code attempting to load a mode module for an editor from 3 different locations. If mode_path is foo, it will try to load foo, then ./modes/foo/foo and then ./modes/foo/foo_mode.

The example at requirejs.org shows how one might handle a case where they want to try multiple locations for a resource they want to make available with a well-known identifier. Presumably the entire code-base in that example requires jQuery by requiring “jquery”. Whatever location jQuery happens to be located at, it becomes available to the whole code-base as “jquery”.

My example does not care about making the mode known to the entire code-base through a well-known identifier because in this specific case there’s no good reason to do so. The onload function stores the module it gets into a variable and the rest of the code base gets it by calling a getMode() method.