Recently one of my friends asked me a question:
![]+1+[]+false
going with my instincts after evaluation I answered 2.
But when I tried the expression in browser console the output I get is "1false".
for me the weird part is 1+[] which evaluates to "1"?
I couldn't understand why is that and how is it working?
Can someone please explain?
Thanks
Adding a number and an object, tries to convert the object to a primitive. To do that, it calls .toString() on the object, the empty array in this case, and for arrays that calls .join(), and [].join() is "", and thus 1 + [] is the same as "1".
Also keep in mind that additions go from left to right.
(![]) + 1 + [] + false // objects are truthy, therefore *not object* is false
(false + 1) + [] + false // boolean gets coerced to number, and false is 0
(0 + 1) + [] + false // 0 + 1 is 1
(1 + []) + false // .valueOf() as described above
(1 + "") + false
"1" + false
"1false"
Also keep in mind that additions go from left to right. Thanks for this
[].valueOf() is [] only. So toString method will be called. Your explanation is not clear why [] coerced to "".Since JavaScript is a weakly-typed language, values can also be converted between different types automatically when you apply operators to values of different type, and it is called implicit type coercion.
In general the algorithm is as follows:
Numeric conversion first calls valueOf (3) with a fallback to toString (2).
String conversion does the opposite: toString (2) followed by valueOf (3).
+ operator triggers numeric conversion.
so valueOf will be called for blank array, which is blank array only. [].valueOf() // [].
Since valueOf is not primitive it will call [].toString() //""
so 1 + [] will convert to 1 + "".
Since one of the operands is string operator triggers string conversion for 1.
so 1 + "" will convert to "1" + ""
And hence you get the output "1"
If at least one operand is string type, the other operand is converted to string and the concatenation is executed.
So in this case 1+[] -
[].toString() gives the empty string and the 1 will be stringified and you get the output "1"
further false also will be stringified and "1"+false = "1false"
+[]gets evaluated to''because the + converts the empty array to a string. So your expression is equivalent to:![]+1+''+falsewhich is equivalent tofalse+1+''+false, once again when converted to a string due to + operator, comes out to1false.