我应该使用！==还是==！ 在检查不相等时？

Question

Should I use !== or ==! ? Both work, but what is the standard?

I enter "else" for both cases:

var x;
if (x !== undefined)
{
    alert("if..");
}
else
{
    alert('else..') // <--- here
}


var y;
if (y ==! undefined)
{
    alert("if..");
}
else
{
    alert('else..') // <--- here
}

Answer 1

==! is not a comparison operator in Javascript. When you write foo ==! bar foo ==! bar , you are actually writing foo == !bar , using the unary ! negation operator and the binary == comparison operator. This probably does not do what you want it to:

> 'b' ==! 'a'
false
> 'b' !== 'a'
true

This happens to "work" in the particular situation you describe because !undefined evaluates to true , and y == true for undefined y is false , thus putting you on the else branch.

Answer 2

if (x !== undefined)

checks if x is not undefined, while

if (y ==! undefined)

negates undefiend (which returns !undefined == true ) and then checks it for equality with y