webpack-hello/study_bool.html at master · AITUncle/webpack-hello · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<script type="text/javascript">
	if(!'')
		console.log("'' --> false");
	if(!"")
		console.log('"" --> false');
	var undefinedVar ;
	if(!undefinedVar)
		console.log('undefinedVar --> false');
	if(undefinedVar == undefined)
		console.log("undefinedVar==undefined --> true")
	var nullVar = null;
	if(!nullVar)
		console.log('nullVar --> false');
	if(nullVar == undefined)//*****
		console.log('nullVar==undefined --> true;');
	var zeroNum = 0;
	if(!zeroNum && !0.0)//***** 数字只有0才是false
		console.log('zeroNum 0 or 0.0 --> false');
	var negativeNum = -100;
	if(negativeNum)
		console.log('negativeNum --> true');
	/*if(noDeclarationVar)
		//语法错误，执行时抛出异常：Uncaught ReferenceError: noDeclarationVar is not defined
	*/

	//NaN
	if(10/'a')
		console.log('10/a --> false');
	if(!NaN)
		console.log('NaN --> false');
	//这不是NaN这是Infinity，无穷大，因为null会被转换为0；所以返回:true;
	if( 10/null )//*****
		console.log('10/null --> true', 10/null);


	console.log('=========================================================');


	console.log('"" ==>' + Boolean(""));//false
	console.log("'' ==>" + Boolean(''));//false
	console.log("'abc' ==>" + Boolean('abc'));//true
	console.log("'0' ==>" + Boolean('0'));//true

	console.log("null ==>" + Boolean(null));//false
	console.log("0 ==>" + Boolean(0));//false
	console.log("0.0 ==>" + Boolean(0.0));//false
	console.log("0.01 ==>" + Boolean(0.01));//true
	console.log("1 ==>" + Boolean(1));//true
	console.log("-1 ==>" + Boolean(-1));//true

	console.log('NaN ==>' + Boolean(NaN));//false

	console.log('null==undefined ==> ' + (null == undefined) + ';//undefined值派生自null，所以他们是这里返回true');

	console.log('' + typeof null);//object
	console.log('' + typeof undefined);//undefined


	</script>
</body>
</html>