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
}

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
}

• javascript

• 1 1 !== 2 is true, but 1 ==! 2 is false. ==! is wrong-spaced == and ! – Alma Do Commented Mar 7, 2014 at 9:53
• Related: The negation operator in JS. – Sebastian Simon Commented Mar 17, 2022 at 11:09

2 Answers 2

==! is not a parison operator in Javascript. When you write foo ==! bar, you are actually writing foo == !bar, using the unary ! negation operator and the binary == parison 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.

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