function_waitBy.js

function waitBy(byThis, diffThis = 'undefined', maxWait = 10000){
	return new Promise(function(resolve, reject){
		if( typeof byThis == 'string' ){
			let waitPromise = setInterval(function(){
				if( typeof top[byThis] != diffThis ){
					clearInterval(waitPromise);
					resolve(true);
				};
			}, 10);
			let waitPromiseReject = setTimeout(function(){
				resolve(false);
			}, maxWait);
		}else{
			let waitPromise = setInterval(function(){
				let continuee = true;
				byThis.forEach(function( thisVar, i ){
					if( typeof top[thisVar] == diffThis ){
						continuee = false;
					};
				});
				if( continuee == true ){
					clearInterval(waitPromise);
					resolve(true);
				};
			}, 10);
			let waitPromiseReject = setTimeout(function(){
				resolve(false);
			}, maxWait);
		};

	});
};

/*
$('<script src="https://fastcdn.org/FileSaver.js/1.1.20151003/FileSaver.min.js"></script>').appendTo('head');
$('<script src="https://cdnjs.cloudflare.com/ajax/libs/jszip/3.1.5/jszip.min.js"></script>').appendTo('head');
$('<script src="https://cdnjs.cloudflare.com/ajax/libs/jszip-utils/0.0.2/jszip-utils.min.js"></script>').appendTo('head');

waitJZipUtils = waitBy(['saveAs', 'JSZip', 'JSZipUtdils']);
waitJZipUtils.then(function(val){
	if( val == true ){
		//...
	}else{
		console.error('promise error on load saveAs, JSZip and JSZipUtdils');
	};
});
*/