javascript - Image data not loading in async waterfall -

after uploading image app, base64 image data(basically long string) not load variable during async call.

var async = require('async'); var img_data;  // ... functions grab img_data frontend  console.log(img_data); // works  var add_item = function(){     console.log(img_data); // works     async.waterfall([         function(callback){             console.log(img_data); // doesn't work             settimeout(function(){                 console.log(img_data); // works                 // stuff , save image                 callback(null, 'all done');             }, 1000);         }     ], function(err, result){         // handle err/result     }); }  // if add item btn clicked, add_item(); 

it seems img_data isn't loaded, guess, need wait. how decide time required timeout? (variation in image data size) , in cases when no image uploaded, there time wastage. how wait until image data loaded in variable?