[] + [] = "?
you can take a look at this:
http://www.cnblogs.com/ziyunf.
to put it simply:
[] + [] first calls the valueOf method of [] . If valueOf returns basic type , this primitive type value is used. If the returned type is not a basic type, the valueOf method is discarded and the toString method is called instead.
[].valueOf() => [] //
[].toString() => "" //
so [] + [] = "" + "="
you can write a simple example to verify it:
const a = [] //
a.valueOf = () => 3 // valueOf
a + a // 6
a.valueOf = () => ([1, 2, 3])
a + a // ""
a.toString = () => 'haha'
a + a // 'hahahaha'
a.toString = () => ([1, 2, 3])
a + a // [].toString()+[].toString() === ""see the following test and this link: http://www.cnblogs.com/ziyunf.
this is caused by the implicit conversion of data by js's operators.
The + operator injs will call the toString method on the data of the reference type and convert it to a string
http://www.jsfuck.com/ learn
binary operator'+ 'can add two numbers or do string concatenation
the conversion rules of the plus sign give priority to string concatenation
-the above is extracted from the sixth edition of the Rhino Book P70
this is caused by the rules of the plus operator
unless neither of the operands is a class string.
if the rule is to give priority to numeric arithmetic, then [] + [] is 0.
here implicitly converts both sides of the Operand into strings
and so on: [] = =! []
! Convert [] to a Boolean value
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