CODEWITHSUNDEEP
javascript
JavaSheriff
JavaSheriff

Reputation: 7665

Javascript true +/- true is false?

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

Upvotes: 2

Views: 10106

Answers (4)

Matthew Ciaramitaro
Matthew Ciaramitaro

Reputation: 1179

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);

Upvotes: 2

Robin James Kerrison
Robin James Kerrison

Reputation: 1757

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.

Upvotes: 7

user5364144
user5364144

Reputation:

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.

Upvotes: 1

trk
trk

Reputation: 2228

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

!null === true

This might solve it for you.

Upvotes: 1

Related Questions