in
in Determine whether the string on the left or the attribute that can be converted into a string belongs to the right.
var data = { x: , y: };//Directed direct objectalert("x" in data);//true , x is a property of dataalert( in data);//false , is the property value of data.var arr = [, , ];//Define direct array objectalert( in arr);//true ,arr The index of the array includes,, is one of its [] attributes.alert( in arr);//false ，Not his attribute。
instanceof
instanceof You want the instance on the left to be the type of the object on the right.
var date = new Date(); alert(date instanceof Date);//true alert(date instanceof Object);//true alert(date instanceof Number);//false var date = [, , ]; alert(date instanceof Array);//true alert(date instanceof Object);//true alert(date instanceof Number);//false
delete
delete delete the properties of an object
var o = { x: , y: };
alert(delete ); //true Delete successfullyalert("x" in o);//false x is not an attribute of ovar o = ;
alert(delete );//false
y = ;
alert(delete );//true
alert(delete );//false
summary
delete is a unary operator. delete cannot delete built-in global variables in JavaScript, nor can it delete global variables declared by var, but it can delete global variables that are not declared by var.
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