Why (!+[]+[]+![]). length returns 9 in Javascript?

Question

I just read that article and in it there is this expression:

(!+[]+[]+![]).length

Evaluating this in the interpreter:

> (!+[]+[]+![]).length
9

Good, why?

Answer 1

Step by step:

+[] === 0
!+[] === true
!+[]+[] === "true"
(!+[]+[])+![] === (!+[]+[])+false === "truefalse"
"truefalse".length === 9

It has several type coercions that ultimately cause this result. Detailing the first line above:

+[] ⟶ +"" ⟶ 0

So we have:

!+[] ⟶ !0 ⟶ !false ⟶ true

Moving on:

!+[]+[] ⟶ true + [] ⟶ true + "" ⟶ "true"

That one "true" will be concatenated with:

![]  ⟶ !true ⟶ false

Therefore:

"true" + false ⟶ "truefalse"

Relevant references of the specification:

• Toboolean
• Tonumber
• Array.prototype.toString
• Unary + Operator
• Addition Operator (+)

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About the article that you quoted

I read the article and I was disappointed. It’s okay that it has become a slogan to say that JS sucks, à la Wat, but there are some unfair criticisms. So I’ll list some points here in defense of language:

• When trying to make comparisons between certain types (such as array and Object), the language could even throw an exception. But for me the real WTF is the programmer doing something like {} > [] or ![] and expect an answer that makes sense... Language has chosen to type coercion, and the rules of that are necessarily arbitrary.

• It’s not strange that the kind of NaN be number, since NaN is defined in the floating point number arithmetic standard (IEEE 754)

• "string" instanceof String === false because there is a difference between the primitive values of type string and objects of type String. In Java it is similar, no?

• Complain that 0.1+0.2!==0.3 is basic knowledge of floating point arithmetic in virtually any language.

• Complain that Math.max()<Math.min() is not having read any documentation about these methods. Their goal is not to return the maximum and minimum values understood by language, but between the past. If you pass nothing, max returns -Infinity, which makes some sense.

I could say more, but I’ll stop here...