JavaScript比较

Question

I would like to know why the following comparisons in javascript will give me different results.

(1==true==1)
true

(2==true==2)
false

(0==false==0)
false

(0==false)
true

I can not figure out why.

Answer 1

The tests are equivalent to these:

(true==1)
true

(false==2)
false

(true==0)
false

which is equivalent to these:

(1==1)
true

(0==2)
false

(1==0)
false

In each case the == converts the boolean to a number 1 or 0 . So the first == in each one gives the initial true/false value, which is then used as the first operand for the second == .

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Or to put it all inline:

((1==true)==1)
((1==1)   ==1)
((true)   ==1)
((1)      ==1)
true

((2==true)==2)
((2==1)   ==2)
((false)  ==2)
((0)      ==2)
false

((0==false)==0)
((0==0)    ==0)
((false)   ==0)
((0)       ==0)
false

Answer 2

I think this is because it is parsed as such:

( (0 == false) == 0 )

Which would be saying

( true == false )

Answer 3

Each of this operation is done in two steps.

(1==true==1)

first operation is 1 == true => true

So second is true == 1 => true

(2==true==2)

first operation is 2 == true => false (just number 1 is equivalent to true in js)

So second is false == 2 => true

(0==false==0)

first operation is 0 == false => true

So second is true == 0 => false