When is RequireJS' require call asynchronous? When is it synchronous?

问题

I use RequireJS to load my modules in one of my projects. I see around the web different ways to require modules using the require call (and not define).

Lets assume I have a module named "JQuery" and I would like to require it. Two ways are possible as I saw in examples:

This:

require(["JQuery"], function($){
   $.doSomething();
})

And this:

var $ = require("JQuery");
$.doSomething();

My question is if the load is async as RequireJS documantation says it is, how can the second convention work? How can I say for sure that $ is defined and that the first row completed before the second row executes?

回答1:

RequireJS always loads modules asynchronously but it allow a form of require that looks synchronous. Your second snippet is actually missing some really important code. (Also, the module name for jQuery is hardcoded to jquery. You could write a configuration that allows you to refer to it as jQuery but there's no point.) This form of the require calls is designed to be used inside modules so:

define(function (require) {
    var $ = require("jquery");
    $.doSomething();
});

What RequireJS does with the code above is transform it into this before executing it:

define(['jquery'], function (require) {
    var $ = require("jquery");
    $.doSomething();
});

Note the addition of the dependency as the first argument of define. When RequireJS executes the code, it finds the dependency, loads jquery and then calls the anonymous function. By the time require("jquery") is encountered the module is already loaded. At the end of the day while the require call looks synchronous, the loading of the module it requires still happens asynchronously.

Can you use this synchronous form require outside of a define call? Only if you are okay with failures. This require call will fail if the module passed to it is not already loaded. You get the infamous error:

Module name ... has not been loaded yet for context: ...

Using it in a define like I've shown above is safe. Or I guess you could do:

require(['jquery'], function (require) {
    var $ = require("jquery");
    $.doSomething();
});

which would work but what's the point of manually repeating the dependency. (In case you wonder, RequireJS does not transform a require call with a callback like the one I have in my example here in the same way it transforms a define call as I've shown above.)

回答2:

According to require.js docs it is a wrapper. So it is rebuilding code to work asynchronously. It is called CommonJs wrapper. You can find more info about it here

回答3:

I don't know requirejs. But I think it's possible to load module in both sync and async ways.

function require(name,cb){
    if(cb !== undefined){
        requireAsync(name,cb);
    }else{
        requireSync(name);
    }
}

来源：https://stackoverflow.com/questions/28341390/when-is-requirejs-require-call-asynchronous-when-is-it-synchronous

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javascript

asynchronous

requirejs

require