Stringy objects
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Stringy objects
Solution

We contemplate situations in which string equivalence in Javascript is clearly not what it seems - and explain why it all goes wrong.

 

 

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Background

Javascript makes it easy to add functions that manipulate basic data. It is also true that in Javascript everything is an object.

For example, the humble and over-worked string literal is an object complete with methods and properties. For example:

alert("ABCD".length);

displays 4. This almost seems against the laws of programming when you first see it but yes raw data in the form of a quoted string is an example of a string object and has the methods and properties of a string object.

 

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Puzzle

With this in mind consider the following simple function that tests to see if a string is equal to a target string literal:

function answer(s){
return s==="yes";
};

Notice that this is an exact test. That is, s has to be equal to "Yes" and not convertible to "Yes" from some other data type. In this case using either == which is a looser test for equality, or using === which is strict equality doesn't seem to be a big issue. After all a string is either a string or it isn't and it is either equal to "Yes" or it isn't. So you could use:

function answer(s){
return s=="yes";
};

just as easily and expect to get the same results. But it it is generally held (for example JSLint) that you should avoid == and != so the first function is the one that was used.

The utility function was used as part of a much larger program and eventually a bug was tracked down to it sometimes returning false when the string passed to it was indeed "yes".

The function was carefully checked and two debugging commands added:

function answer(s){
alert(s);
alert(s==="yes");
return s ==="yes";
}

so that the value of s could be seen along with the result. During one run the function returned:

yes

and

false

That is, the parameter passed in stored "yes" and  the result of the comparison s==="yes" was false.

This cannot be!

What is going on?

Turn to the next page when you are ready to find out.

 

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