js隐式转换isNaN(num)

var a = 123;
console.log( isNaN( a ) );
//控制台打印false
//isNaN(num)方法为判断num是否为NaN,如果是则返回true,否者返回false

var a = NaN;
console.log( isNaN( a ) );
//控制台打印为true

var a = undefined;
console.log( isNaN( a ) );
//控制台打印为true;undefined明显不等于NaN,那么为啥结果为true了？

因为isNaN(num)判断时先将num隐式转化Number(num)，在于NaN比较

var a = undefined;
//Number(undefined),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

var a = "aaa";
//Number(a),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

var a = "";
//Number(a),结果为0
console.log( isNaN( a ) );
//控制台打印为false

var a = true;
//Number(a),结果为1
console.log( isNaN( a ) );
//控制台打印为false

var a = "123ad";
//Number(a),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

var a = "123";
//Number(a),结果为123
console.log( isNaN( a ) );
//控制台打印为false

var a = window;
//Number(a),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

var a = null;
//Number(a),结果为0
console.log( isNaN( a ) );
//控制台打印为false

var a = Object;
//Number(a),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

var a = [1,2,3];
//Number(a),结果为NaN
console.log( isNaN( a ) );
//控制台打印为true

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