What is this communicating: my_var = my_var || 69 [duplicate]

Question

I saw this in a Javascript example

my_var = my_var || 69

I assume it means to check if my_var exists, if not set my_var to 69. Is this the case? Is there any documentation on this, it is very hard to represent as a google/SO search, could somebody point me in the direction of docs or duplicate QA?

(The example didn't use 69, that's just me being crass)

Answer 1

Easy enough to try in the JS console.

var my_var
my_var = my_var || 69
//69

var my_var = 5
my_var = my_var || 69
//5

You are setting the variable only if it is currently carrying a falsy value.

Falsy values in JS are:

1. false
2. null
3. undefined
4. The empty string ''
5. The number 0
6. The number NaN