what is this javascript expression for

Question

I have worked with javascript during 2 years now, but I have never seen an expression like this one. in google chrome console I have typed this

var a=456;
var b=789;

then I typed this

a|=b 

and the result was 989

can someone tell me what is this expression for, and why is the result 989?

Answer 1

That is a bitwise OR operation. When used in that fashion it is "or equals" where the result is assigned to the variable.

 111001000 //456
1100010101 //789
1111011101 //989

Answer 2

That expression is called bitwise or, with assignment. It takes the each individual bit position of each number, and returns a 1 for that bit if it is 1 at that position in either of the numbers, otherwise assigning 0 if both are 0.

See the Mozilla documentation for other such bitwise operations.

It is more commonly used in systems languages like C and C++.

Answer 3

well the expression

a|=b;

is like when you do a+=b; it's equivalent to

a = a | b;

the operator | is the OR, not like the operator ||. this one is used as the operator OR for binary operations

5   |  1   ->     0101 | 0001          ->      0101    ->    5

in your case

456 | 789  ->  111001000 | 1100010101  ->   1111011101  ->    989