JavaScript true +/- true是否为false？

Question

I am trying to get an expression to test if one field is null and treat is as false

a=true
b=true
-------------
true

 a=null
 b=true
 -------------
 true

but when I execute:

var a=true;
var b=true;
alert((a+b) == true);   => false

It returns false, i don't get it.

var a=null;
var b=true;
alert((a+b) == true);   => true

Answer 1

The general solution for this in javascript is to use !! to parse to a boolean. !! negates the truthiness twice, resulting in a boolean which has the same truthiness of the original.

You should then use && as a logical and operation.

var a=null;
var b=true;
console.log(!!a && !!b);   // false

Edit: An addendum on the strange + behaviour

The strangeness you're seeing when using + instead of && is because, in JavaScript, + coerces booleans to integers, with true becoming 1 and false becoming 0 .

Hence

true + true \\ 2
true + false \\ 1

And then when doing

true + true == true

the left-hand-side of the equality comparison resolves to 2 , JavaScript then coerces the right-hand-side to 1 and thus the equality check fails.

When doing

null + true == true

the left-hand-side becomes the integer 1 , and then so does the right.

I'd recommend reading the MDN guide on Equality comparisons and sameness for more on JavaScript's value coercion and abstract equality checks.

Answer 2

 var a = true; var b = true; console.log((a & b) === 1); var c = null; console.log((a & c) === 1); 

true == 1 This is important. When you convert true to a number, it will be 1

then true + true == true becomes 2 == 1 which is false

similarly null + true == true becomes 1 == 1 which is true because null resolves to 0

I think you are going about testing for null the wrong way.

try

alert((a & b) === 1); 

Answer 3

For null and true to test to true you could probably do:

!null === true

This might solve it for you.

Answer 4

It is simple. When you are trying to make + operation true is like 1 and false is like 0. Then if you make true+true you get 2 and two is not 1, so it is not true .