'\n\t\r' == 0 is true?

Question

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

What I did is:

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

And that really annoy me. So I did more:

var txt = "\r";  //"return" txt == 0;        //true  var txt = "\t";  //"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 ="\n\t\r"; 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/

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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 "\n" ("\r", "\t") 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.

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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.

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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.

Answer 2

Because JavaScript is a loosely typed language, it attempts to type cast your 1st side of the comparison to the other so that they would match each other.

Any string which does not contain a number, becomes 0 when compared to an integer, and becomes true (Except in certain situations), when compared to a Boolean.

Light reading material.