This codes retreives an existing blob URL with sas with permissions to write an Azure Blob, but this time using javascript from Node.js, working on Azure Mobile Services

GetSasFromURL_AMS.js

var azure = require('azure');
var blobService = azure.createBlobService('storageAccountName', 'storageAccountKey', 'host');
blobService.createContainerIfNotExists(containerName, {
  		    publicAccessLevel : 'blob'
	      }, function(error) {
	  	    if (error) {
		          console.log("Error : " + error);
		      }
	     });
var mins = 10;
var Constants = azure.Constants;
var BlobConstants = Constants.BlobConstants;
var startDate = new Date();
var expiryDate = new Date(startDate.getTime());
expiryDate.setMinutes(startDate.getMinutes() + mins);
var sharedAccessPolicy = {
  AccessPolicy : {
      Permissions : BlobConstants.SharedAccessPermissions.WRITE,
      Start : startDate,
      Expiry : expiryDate
  }
};
var signature = blobService.generateSharedAccessSignature(containerName, blobName, sharedAccessPolicy);
var urlSignature = signature.url();