Load one-script-after another

Question

I have this simple function which loads scripts into the current DOM:

function loadscripts ( async ) {
    if( async === undefined ) {
        async = false;
    }

    var scripts = [];
    var _scripts = ['jquery.min.js', 'bootstrap.min.js', 'plugins.js', 'main.js'];

    for(var s in _scripts) {
        scripts[s] = document.createElement('script');
        scripts[s].type = 'text/javascript';
        scripts[s].src = _scripts[s];
        scripts[s].async = async;

        document.getElementsByTagName('head').appendChild( scripts[s] );
    }
}

They are loaded fine and without no errors. I know there are event handlers when loading scripts programmatically:

• onreadystatechange, and
• onload, etc

Now I would like to wish to do the following:

• Load first script from the array, and when the event handlers are COMPLETE load the next one, and so on (recursively).

---

Sorry, I have not provided the onreadystatechange and onload events in my function.

Answer 1

You could do it with promises like this.

// loads an individual script
var loadScript = function (path) {
  // generate promise
  return new Promise(function (fulfill, reject) {
  // create object
    var script = document.createElement('script');
    
    // when it loads or the ready state changes
    script.onload = script.onreadystatechange = function () {
      // make sure it's finished, then fullfill the promise
      if (!this.readyState || this.readyState == 'complete')
        fulfill(this);
    };

    // begin loading it
    script.src = path;

    // add to head
    document.getElementsByTagName('head')[0].appendChild(script);
  });
};

// this is the one you want
var loadScripts = function (scripts) {
  return scripts.reduce(
    function (queue, path) {
      // once the current item on the queue has loaded, load the next one
      return queue.then(function () {
        // individual script
        return loadScript(path);
      });
    },
    Promise.resolve() /* this bit is so queue is always a promise */
  );
};

Usage:

loadScripts(["foo.js", "bar.js"]).then(function () {
  // whatever you want to happen when it's all done
});

It doesn't do any error handling though, so you'll have to implement that it if you need it.

Answer 2

We can load one script after another in html by using lab.min.js.

This library run in Synchronous way.

<script>
var _DIR_ = "js/vendor/";

    $LAB.setOptions({
        AlwaysPreserveOrder: true
    });
    $LAB
        .queueScript(_DIR_ + 'jquery-v2-1-3.min.js')
        .queueWait()
        .queueScript(_DIR_ + 'angular.min.js')
        .queueWait()
        .queueScript(_DIR_ + 'socket.io-1.3.5.js')
        .queueScript(_DIR_ + 'angular-bootstrap/ui-bootstrap-tpls.min.js')
        .queueScript(_DIR_ + 'Angular plugins/angular-ui-notification.min.js')
        .runQueue();
</script>

Answer 3

Had to load scripts in given order x,y,z and since I only had a few of them. This approach worked for me. Could not load these in body or head since they are quite big and I'm using a static landing page.

document.addEventListener('DOMContentLoaded', function(event) {
        // initial DOM loaded
        var vendorJS = document.createElement('script');
        vendorJS.src = 'assets/vendor.js';

        var appJS = document.createElement('script');
        appJS.src = 'assets/application.js';

        var envJS = document.createElement('script');
        envJS.src = 'assets/env.js';

        document.body.appendChild(vendorJS);

        vendorJS.onload = vendorJS.onreadystatechange = function() {
            document.body.appendChild(appJS);

            appJS.onload = appJS.onreadystatechange = function() {
                document.body.appendChild(envJS);
            };
        };
});

Answer 4

I would do this that way :

LoadScripts();

function LoadScripts(async)
{
    if( async === undefined ) {
        async = false;
    }
    var scripts = [];
    var _scripts = ['jquery.min.js', 'bootstrap.min.js', 'plugins.js', 'main.js'];

    if(async){
        LoadScriptsAsync(_scripts, scripts)
    }else{
        LoadScriptsSync(_scripts,scripts)
    }
}

// what you are looking for :
function LoadScriptsSync (_scripts, scripts) {

    var x = 0;
    var loopArray = function(_scripts, scripts) {
        // call itself
        loadScript(_scripts[x], scripts[x], function(){
            // set x to next item
            x++;
            // any more items in array?
            if(x < _scripts.length) {
                loopArray(_scripts, scripts);   
            }
        }); 
    }
    loopArray(_scripts, scripts);      
}

// async load as in your code
function LoadScriptsAsync (_scripts, scripts){
    for(var i = 0;i < _scripts.length;i++) {
        loadScript(_scripts[i], scripts[i], function(){});
    }
}

// load script function with callback to handle synchronicity 
function loadScript( src, script, callback ){

    script = document.createElement('script');
    script.onerror = function() { 
        // handling error when loading script
        alert('Error to handle')
    }
    script.onload = function(){
        console.log(src + ' loaded ')
        callback();
    }
    script.src = src;
    document.getElementsByTagName('head')[0].appendChild(script);
}

Answer 5

Try this:

function loadscripts ( async ) {
    if( async === undefined ) {
        async = false;
    }

    var scripts = [];
    var _scripts = ['jquery.min.js', 'bootstrap.min.js', 'plugins.js', 'main.js'];
    for(var s in _scripts) {
        scripts[s] = document.createElement('script');
        scripts[s].type = 'text/javascript';
        scripts[s].src = _scripts[s];
        scripts[s].async = async;
    }
    var loadNextScript = function() {
        var script = scripts.shift();
        var loaded = false;
        document.getElementsByTagName('head').appendChild( script );
        script.onload = script.onreadystatechange = function() {
            var rs = this.readyState;
            if (rs && rs != 'complete' && rs != 'loaded') return;
            if (loaded) return;
            loaded = true;
            if (scripts.length) {
                loadNextScript();
            } else {
                // done
            }
        };
    };
    loadNextScript();
}