javascript - ' ' == 0 is true?

Question

Today when I was doing some experiments with ==, I accidentally found out that " " == 0. How on earth does " " equal to 0, or false?

What I did is:

var txt = "
";  //new line
txt == 0;        //it gives me true

And that really annoy me. So I did more:

var txt = "
";  //"return"
txt == 0;        //true

var txt = "";  //"tab"
txt == 0;        //true

It does not make sense, at all. How's that happen? And more crazy is this:

//Checking for variable declared or not

var txt ="

";
if(txt!=false){
    console.log("Variable is declared.");
}else{
    console.log("Variable is not declared.");
}

What it gives me is Variable is not declared.

How is it equal to 0, or false???

Answer 1

This behaviour might be surprising but can be explained by having a look at the specification.

We have to look at the what happens when a comparison with the equals operator is performed. The exact algorithm is defined in section 11.9.3.

I built a simple tool to demonstrate which algorithm steps are executed: https://felix-kling.de/js-loose-comparison/

---

string == integer

The step we have to look at is #5:

5. If Type(x) is String and Type(y) is Number,
return the result of the comparison ToNumber(x) == y.

That means the string " " (" ", "") is converted to a number first and then compared against 0.

How is a string converted to a number? This is explained in section 9.3.1. In short, we have:

The MV (mathematical value) of StringNumericLiteral ::: StrWhiteSpace is 0.

where StrWhiteSpace is defined as

StrWhiteSpace :::
    StrWhiteSpaceChar StrWhiteSpace_opt

StrWhiteSpaceChar :::
    WhiteSpace
    LineTerminator

This just means that the numerical value of strings containing white space characters and/or a line terminator is 0.
Which characters are considered as white space characters is defined in section 7.3.

---

string == boolean

The step we have to look at is #7:

7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

How booleans are converted to numbers is pretty simple: true becomes 1 and false becomes 0.

Afterwards we are comparing a string against a number, which is explained above.

---

As others have mentioned, strict comparison (===) can be used to avoid this "problem". Actually you should only be using the normal comparison if you know what you are doing and want this behaviour.