Difference between || and ||=? [duplicate]

Answer 1

||= will set the left-hand value to the right hand value only if the left-hand value is falsey.

In this case, both 6 and 4 are truthy, so a = 6 || 4 will set a to the first truthy value, which is 6.

a ||= 6 will set a to 6 only if a is falsey. That is, if it's nil or false.

a = nil
a ||= 6
a ||= 4
a # => 6

Answer 2

x ||= y means assigning y to x if x is null or undefined or false ; it is a shortcut to x = y unless x.

With Ruby short-circuit operator || the right operand is not evaluated if the left operand is truthy.

Now some quick examples on my above lines on ||= :

when x is undefined and n is nil:

with unless

y = 2
x = y unless x
x # => 2

n = nil
m = 2
n = m unless n
m # => 2

with =||

y = 2
x ||= y
x # => 2

n = nil
m = 2
n ||= m
m # => 2

Answer 3

a ||= 6 only assigns 6 if it wasn't already assigned. ( actually, falsey, as Chris said)

a = 4
a ||= 6
=> 4

a = 4 || 6
=> 4

Answer 4

You can expand a ||= 6 as

a || a = 6

So you can see that it use a if a is not nil or false, otherwise it will assign value to a and return that value. This is commonly used for memoization of values.

Update

Thanks to the first comment for pointing out the true expansion of the ||= (or equal) operator. I learned something new and found this interesting post that talks about it. http://dablog.rubypal.com/2008/3/25/a-short-circuit-edge-case

Answer 5

Both expressions a = 6 || 4 and a ||= 6 return the same result but the difference is that ||= assigns value to variable if this variable is nil or false.