Dynamically load external Javascript file, and wait for it to load - without using JQuery

I'm trying to dynamically load an external .js file (according to an input I get from the user). Currently my code looks like this:

function createScript(src, id) {
    if (document.getElementById(id) == null) {
        var newScript = document.createElement('script');
        newScript.setAttribute("type", "text/javascript");
        newScript.setAttribute("src", src);
        newScript.setAttribute("id", id);
        document.getElementsByTagName("head")[0].appendChild(newScript);
    }
}

This kind of work. It does load the script, but if I try to call a function that is defined inside the new script it won't work. The weird thing is if I wait for a second (using an alert("test") or something like that) the script does work.

I guess its because the browser adds the <script> tag immediately, but it takes it a little longer to "read" the script inside of the .js file. I tried playing with "typeof testfunction === 'function'" with a few variations but no luck.

Any good way of doing this?

February 21, 2017 - how jQuery does it

jQuery.getScript is now just a wrapper for jQuery.get

getScript: function( url, callback ) {
    return jQuery.get( url, undefined, callback, "script" );
}

jQuery.get is just a wrapper of jQuery.ajax – it is defined using metaprogramming as ...

jQuery.each( [ "get", "post" ], function( i, method ) {
    jQuery[ method ] = function( url, data, callback, type ) {

        // Shift arguments if data argument was omitted
        if ( jQuery.isFunction( data ) ) {
            type = type || callback;
            callback = data;
            data = undefined;
        }

        // The url can be an options object (which then must have .url)
        return jQuery.ajax( jQuery.extend( {
            url: url,
            type: method,
            dataType: type,
            data: data,
            success: callback
        }, jQuery.isPlainObject( url ) && url ) );
    };
} );

jQuery.ajax is this 430+ LOC monster

ajax: function( url, options ) {

  // If url is an object, simulate pre-1.5 signature
  if ( typeof url === "object" ) {
    options = url;
    url = undefined;
  }

  // Force options to be an object
  options = options || {};

  var transport,

    // URL without anti-cache param
    cacheURL,

    // Response headers
    responseHeadersString,
    responseHeaders,

    // timeout handle
    timeoutTimer,

    // Url cleanup var
    urlAnchor,

    // Request state (becomes false upon send and true upon completion)
    completed,

    // To know if global events are to be dispatched
    fireGlobals,

    // Loop variable
    i,

    // uncached part of the url
    uncached,

    // Create the final options object
    s = jQuery.ajaxSetup( {}, options ),

    // Callbacks context
    callbackContext = s.context || s,

    // Context for global events is callbackContext if it is a DOM node or jQuery collection
    globalEventContext = s.context &&
      ( callbackContext.nodeType || callbackContext.jquery ) ?
        jQuery( callbackContext ) :
        jQuery.event,

    // Deferreds
    deferred = jQuery.Deferred(),
    completeDeferred = jQuery.Callbacks( "once memory" ),

    // Status-dependent callbacks
    statusCode = s.statusCode || {},

    // Headers (they are sent all at once)
    requestHeaders = {},
    requestHeadersNames = {},

    // Default abort message
    strAbort = "canceled",

    // Fake xhr
    jqXHR = {
      readyState: 0,

      // Builds headers hashtable if needed
      getResponseHeader: function( key ) {
        var match;
        if ( completed ) {
          if ( !responseHeaders ) {
            responseHeaders = {};
            while ( ( match = rheaders.exec( responseHeadersString ) ) ) {
              responseHeaders[ match[ 1 ].toLowerCase() ] = match[ 2 ];
            }
          }
          match = responseHeaders[ key.toLowerCase() ];
        }
        return match == null ? null : match;
      },

      // Raw string
      getAllResponseHeaders: function() {
        return completed ? responseHeadersString : null;
      },

      // Caches the header
      setRequestHeader: function( name, value ) {
        if ( completed == null ) {
          name = requestHeadersNames[ name.toLowerCase() ] =
            requestHeadersNames[ name.toLowerCase() ] || name;
          requestHeaders[ name ] = value;
        }
        return this;
      },

      // Overrides response content-type header
      overrideMimeType: function( type ) {
        if ( completed == null ) {
          s.mimeType = type;
        }
        return this;
      },

      // Status-dependent callbacks
      statusCode: function( map ) {
        var code;
        if ( map ) {
          if ( completed ) {

            // Execute the appropriate callbacks
            jqXHR.always( map[ jqXHR.status ] );
          } else {

            // Lazy-add the new callbacks in a way that preserves old ones
            for ( code in map ) {
              statusCode[ code ] = [ statusCode[ code ], map[ code ] ];
            }
          }
        }
        return this;
      },

      // Cancel the request
      abort: function( statusText ) {
        var finalText = statusText || strAbort;
        if ( transport ) {
          transport.abort( finalText );
        }
        done( 0, finalText );
        return this;
      }
    };

  // Attach deferreds
  deferred.promise( jqXHR );

  // Add protocol if not provided (prefilters might expect it)
  // Handle falsy url in the settings object (#10093: consistency with old signature)
  // We also use the url parameter if available
  s.url = ( ( url || s.url || location.href ) + "" )
    .replace( rprotocol, location.protocol + "//" );

  // Alias method option to type as per ticket #12004
  s.type = options.method || options.type || s.method || s.type;

  // Extract dataTypes list
  s.dataTypes = ( s.dataType || "*" ).toLowerCase().match( rnothtmlwhite ) || [ "" ];

  // A cross-domain request is in order when the origin doesn't match the current origin.
  if ( s.crossDomain == null ) {
    urlAnchor = document.createElement( "a" );

    // Support: IE <=8 - 11, Edge 12 - 13
    // IE throws exception on accessing the href property if url is malformed,
    // e.g. http://example.com:80x/
    try {
      urlAnchor.href = s.url;

      // Support: IE <=8 - 11 only
      // Anchor's host property isn't correctly set when s.url is relative
      urlAnchor.href = urlAnchor.href;
      s.crossDomain = originAnchor.protocol + "//" + originAnchor.host !==
        urlAnchor.protocol + "//" + urlAnchor.host;
    } catch ( e ) {

      // If there is an error parsing the URL, assume it is crossDomain,
      // it can be rejected by the transport if it is invalid
      s.crossDomain = true;
    }
  }

  // Convert data if not already a string
  if ( s.data && s.processData && typeof s.data !== "string" ) {
    s.data = jQuery.param( s.data, s.traditional );
  }

  // Apply prefilters
  inspectPrefiltersOrTransports( prefilters, s, options, jqXHR );

  // If request was aborted inside a prefilter, stop there
  if ( completed ) {
    return jqXHR;
  }

  // We can fire global events as of now if asked to
  // Don't fire events if jQuery.event is undefined in an AMD-usage scenario (#15118)
  fireGlobals = jQuery.event && s.global;

  // Watch for a new set of requests
  if ( fireGlobals && jQuery.active++ === 0 ) {
    jQuery.event.trigger( "ajaxStart" );
  }

  // Uppercase the type
  s.type = s.type.toUpperCase();

  // Determine if request has content
  s.hasContent = !rnoContent.test( s.type );

  // Save the URL in case we're toying with the If-Modified-Since
  // and/or If-None-Match header later on
  // Remove hash to simplify url manipulation
  cacheURL = s.url.replace( rhash, "" );

  // More options handling for requests with no content
  if ( !s.hasContent ) {

    // Remember the hash so we can put it back
    uncached = s.url.slice( cacheURL.length );

    // If data is available, append data to url
    if ( s.data ) {
      cacheURL += ( rquery.test( cacheURL ) ? "&" : "?" ) + s.data;

      // #9682: remove data so that it's not used in an eventual retry
      delete s.data;
    }

    // Add or update anti-cache param if needed
    if ( s.cache === false ) {
      cacheURL = cacheURL.replace( rantiCache, "$1" );
      uncached = ( rquery.test( cacheURL ) ? "&" : "?" ) + "_=" + ( nonce++ ) + uncached;
    }

    // Put hash and anti-cache on the URL that will be requested (gh-1732)
    s.url = cacheURL + uncached;

  // Change '%20' to '+' if this is encoded form body content (gh-2658)
  } else if ( s.data && s.processData &&
    ( s.contentType || "" ).indexOf( "application/x-www-form-urlencoded" ) === 0 ) {
    s.data = s.data.replace( r20, "+" );
  }

  // Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
  if ( s.ifModified ) {
    if ( jQuery.lastModified[ cacheURL ] ) {
      jqXHR.setRequestHeader( "If-Modified-Since", jQuery.lastModified[ cacheURL ] );
    }
    if ( jQuery.etag[ cacheURL ] ) {
      jqXHR.setRequestHeader( "If-None-Match", jQuery.etag[ cacheURL ] );
    }
  }

  // Set the correct header, if data is being sent
  if ( s.data && s.hasContent && s.contentType !== false || options.contentType ) {
    jqXHR.setRequestHeader( "Content-Type", s.contentType );
  }

  // Set the Accepts header for the server, depending on the dataType
  jqXHR.setRequestHeader(
    "Accept",
    s.dataTypes[ 0 ] && s.accepts[ s.dataTypes[ 0 ] ] ?
      s.accepts[ s.dataTypes[ 0 ] ] +
        ( s.dataTypes[ 0 ] !== "*" ? ", " + allTypes + "; q=0.01" : "" ) :
      s.accepts[ "*" ]
  );

  // Check for headers option
  for ( i in s.headers ) {
    jqXHR.setRequestHeader( i, s.headers[ i ] );
  }

  // Allow custom headers/mimetypes and early abort
  if ( s.beforeSend &&
    ( s.beforeSend.call( callbackContext, jqXHR, s ) === false || completed ) ) {

    // Abort if not done already and return
    return jqXHR.abort();
  }

  // Aborting is no longer a cancellation
  strAbort = "abort";

  // Install callbacks on deferreds
  completeDeferred.add( s.complete );
  jqXHR.done( s.success );
  jqXHR.fail( s.error );

  // Get transport
  transport = inspectPrefiltersOrTransports( transports, s, options, jqXHR );

  // If no transport, we auto-abort
  if ( !transport ) {
    done( -1, "No Transport" );
  } else {
    jqXHR.readyState = 1;

    // Send global event
    if ( fireGlobals ) {
      globalEventContext.trigger( "ajaxSend", [ jqXHR, s ] );
    }

    // If request was aborted inside ajaxSend, stop there
    if ( completed ) {
      return jqXHR;
    }

    // Timeout
    if ( s.async && s.timeout > 0 ) {
      timeoutTimer = window.setTimeout( function() {
        jqXHR.abort( "timeout" );
      }, s.timeout );
    }

    try {
      completed = false;
      transport.send( requestHeaders, done );
    } catch ( e ) {

      // Rethrow post-completion exceptions
      if ( completed ) {
        throw e;
      }

      // Propagate others as results
      done( -1, e );
    }
  }

  // Callback for when everything is done
  function done( status, nativeStatusText, responses, headers ) {
    var isSuccess, success, error, response, modified,
      statusText = nativeStatusText;

    // Ignore repeat invocations
    if ( completed ) {
      return;
    }

    completed = true;

    // Clear timeout if it exists
    if ( timeoutTimer ) {
      window.clearTimeout( timeoutTimer );
    }

    // Dereference transport for early garbage collection
    // (no matter how long the jqXHR object will be used)
    transport = undefined;

    // Cache response headers
    responseHeadersString = headers || "";

    // Set readyState
    jqXHR.readyState = status > 0 ? 4 : 0;

    // Determine if successful
    isSuccess = status >= 200 && status < 300 || status === 304;

    // Get response data
    if ( responses ) {
      response = ajaxHandleResponses( s, jqXHR, responses );
    }

    // Convert no matter what (that way responseXXX fields are always set)
    response = ajaxConvert( s, response, jqXHR, isSuccess );

    // If successful, handle type chaining
    if ( isSuccess ) {

      // Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
      if ( s.ifModified ) {
        modified = jqXHR.getResponseHeader( "Last-Modified" );
        if ( modified ) {
          jQuery.lastModified[ cacheURL ] = modified;
        }
        modified = jqXHR.getResponseHeader( "etag" );
        if ( modified ) {
          jQuery.etag[ cacheURL ] = modified;
        }
      }

      // if no content
      if ( status === 204 || s.type === "HEAD" ) {
        statusText = "nocontent";

      // if not modified
      } else if ( status === 304 ) {
        statusText = "notmodified";

      // If we have data, let's convert it
      } else {
        statusText = response.state;
        success = response.data;
        error = response.error;
        isSuccess = !error;
      }
    } else {

      // Extract error from statusText and normalize for non-aborts
      error = statusText;
      if ( status || !statusText ) {
        statusText = "error";
        if ( status < 0 ) {
          status = 0;
        }
      }
    }

    // Set data for the fake xhr object
    jqXHR.status = status;
    jqXHR.statusText = ( nativeStatusText || statusText ) + "";

    // Success/Error
    if ( isSuccess ) {
      deferred.resolveWith( callbackContext, [ success, statusText, jqXHR ] );
    } else {
      deferred.rejectWith( callbackContext, [ jqXHR, statusText, error ] );
    }

    // Status-dependent callbacks
    jqXHR.statusCode( statusCode );
    statusCode = undefined;

    if ( fireGlobals ) {
      globalEventContext.trigger( isSuccess ? "ajaxSuccess" : "ajaxError",
        [ jqXHR, s, isSuccess ? success : error ] );
    }

    // Complete
    completeDeferred.fireWith( callbackContext, [ jqXHR, statusText ] );

    if ( fireGlobals ) {
      globalEventContext.trigger( "ajaxComplete", [ jqXHR, s ] );

      // Handle the global AJAX counter
      if ( !( --jQuery.active ) ) {
        jQuery.event.trigger( "ajaxStop" );
      }
    }
  }

  return jqXHR;
}

So yeah, this is obviously ridiculous to try to remove all the dependencies from this code. You're better off just including jQuery if you want to use jQuery's method for loading external scripts asynchronously.

Or, consider using a different tool altogether.

July 16, 2013 - from jQuery guts with no dependencies ^{[source code citation needed]}

function getScript(src, callback) {
  var s = document.createElement('script');
  s.src = src;
  s.async = true;
  s.onreadystatechange = s.onload = function() {
    if (!callback.done && (!s.readyState || /loaded|complete/.test(s.readyState))) {
      callback.done = true;
      callback();
    }
  };
  document.querySelector('head').appendChild(s);
}

One of the best patterns to use here is to use addEventListener to listen for a custom event such as "MyScriptLoaded". Then, modify your included script to fire this event as its last action. This will ensure that you take action only after the new script has fully loaded.

If you have control over the source of the javascript that gets loaded, your best bet would be to have that code execute a pseudo-callback function that exists within your page context. This is something like the method used for JSONP. So, imagine on your calling page, you have this:

function createScript(src, id) {
    if (document.getElementById(id) == null) {
        var newScript = document.createElement('script');
        newScript.setAttribute("type", "text/javascript");
        newScript.setAttribute("src", src);
        newScript.setAttribute("id", id);
        document.getElementsByTagName("head")[0].appendChild(newScript);
    }
}

function callbackFunc()
{
  // make use of new functions here....
}

And each of your source files might end with this line:

callbackFunc();