How should this program be understood? What exactly is the output? _html/css_WEB-ITnose

var x; var y = null; var z = undefined; document.write( "x:" x " "); document.write("y:" y " "); document.write("z:" z " "); if (x == null) { document.write(111); }

if (x == y) { document.write(222); }

if (x == undefined) { document.write(333); } //-- >

Reply to discussion (solution) Who wrote the code? 78 suffered from chaos. It’s not function(), and the following if is of no use at all.

From the reference book, the result indicated in the reference book is: 111; but the result I ran is: 111222333

If your code is exactly the same as the one in the book, then What this book says is wrong. Refer to the tutorial on W3SCHOOL, which explains the Null and Undefined types more thoroughly.

http://www.cainiao8.com/web/js_blueidea/29_undefined_null.html

Of course, you don’t need to write a function if you call it directly. var x; means that the x variable is defined. But it is not initialized, the system will automatically give it a special value undefined (meaning undefined),

var y = null; The Null type is a data type with only one value, and the special value is null, which Represents a null object reference as well as a pointer.

var z = undefined; means explicitly assigning undefined to a variable.

undefined is derived from null, that is, undefined == null returns true, but is equal to "three =" undefined === null returns false, because their data types are different

So the three if statements will be executed,

Just find a tutorial to learn about javascript data types The explanation becomes