What does “var FOO = FOO || {}” (assign a variable or an empty object to that variable) mean in Javascript?

Question

Looking at an online source code I came across this at the top of several source files.

var FOO = FOO || {};
FOO.Bar = …;

But I have no ide

Answer 1

Another common use for || is to set a default value for an undefined function parameter also:

function display(a) {
  a = a || 'default'; // here we set the default value of a to be 'default'
  console.log(a);
}

// we call display without providing a parameter
display(); // this will log 'default'
display('test'); // this will log 'test' to the console

The equivalent in other programming usually is:

function display(a = 'default') {
  // ...
}