What is the role of character || in the variable declaration in JavaScript?

Question

For example

var myVar = myVar || {};

or

var myVar = myVar || [];

What does this statement mean ?

Answer 1

It provides the default value for myVar in case myVar evaluates to false.

This can happen when myVar is either:

• false
• 0
• empty string
• null
• undefined
• NaN

Answer 2

"OR" which is used to assign a default value. An undefined value evaluates to false, hence "OR"ing it with a value returns the value, and that gets assigned to the variable.

function myNameFunction(theName)
{
   var theName = theName || "John";
   alert("Hello " + theName);
}

myNameFunction("dhruv")  // alerts "Hello dhruv"
myNameFunction()   // alerts "Hello John"

Answer 3

It's for given a default value, and the notation is called OR, as you know it from if statements etc:

Consider this scenario:

var Person = function(age){
     this.age = age;
}
console.log(new Person(20).age);
// Output will be 20.

console.log(new Person().age);
// Output will be undefined.

If you didn't give an age, the output would be undefined.

You can set a default value, if the value you want supplied doesn't exist.

var Person = function(age){
     this.age = age || 0;
}
console.log(new Person(20).age);
// Output will be 20.

console.log(new Person().age);
// Output will be 0.

To know more about when it applies, see @soulchecks answer.