javascript当中null和?developer/article/1517341/undefined的==和===的比较
javascript当中null和?developer/article/1517341/undefined的==和===的比较
例 3.1.3（null和?developer/article/1517341/undefined的==和===的比较） <head> <meta http-equiv="content-type" content="text/html; charset=utf-8"/> </head> <script> //马克-to-win：声明变量x /* if a value is not set, its typeof is ?developer/article/1517341/undefined, its value is ?developer/article/1517341/undefined, if a value= null, its typeof is object, its value is null,but when you use == to test, they are the same, but when to use === to test, they are not the same,== means as long as value is the same, ok, but === means type also must be equal. */ var x; var z1 = "d"; var y = null; // document.writeln("z1 is"+z1); /*if you cancel commenting out the following statement, it will give the blank page, which means it has error.*/ // document.writeln(zyx); document.writeln(x + " is x"); document.writeln(typeof(x) + " is typeof(x)"); document.writeln(y + " is y"); document.writeln(typeof(y) + " is typeof(y)"); var z = ?developer/article/1517341/undefined; document.writeln(z + " is z"); document.writeln(typeof(z) + " is typeof(z)"); if (y == ?developer/article/1517341/undefined) { document.writeln('null and ?developer/article/1517341/undefined is interchangable'); } if (z1 != null) { document.writeln('z1 != null'); } if (y === ?developer/article/1517341/undefined) { document.writeln('null and ?developer/article/1517341/undefined is exactly the same'); } if (x == ?developer/article/1517341/undefined) { document.writeln('声明变量后默认值为?developer/article/1517341/undefined'); } if (x === ?developer/article/1517341/undefined) { document.writeln('声明变量后默认值为exactly the same as ?developer/article/1517341/undefined'); } if (x == null) { document.writeln('声明变量后默认值为null'); }
更多请见：https://blog.csdn.net/qq_44594249/article/details/99689140
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