Page 192 - JavaScript

}

console.log( foo('burger') ); // burger
console.log( foo(100) ); // 100
console.log( foo([]) ); // []
console.log( foo(0) ); // default
console.log( foo(undefined) ); // default

Just keep in mind that for arguments, 0 and (to a lesser extent) the empty string are also often
valid values that should be able to be explicitly passed and override a default, which, with this
pattern, they won’t (because they are falsy).

Null and Undefined

The differences between null and undefined

null and undefined share abstract equality == but not strict equality ===,

null == undefined // true
null === undefined // false

They represent slightly different things:

• undefined represents the absence of a value, such as before an identifier/Object property has
been created or in the period between identifier/Function parameter creation and it's first set,
if any.
• null represents the intentional absence of a value for an identifier or property which has
already been created.

They are different types of syntax:

• undefined is a property of the global Object, usually immutable in the global scope. This
means anywhere you can define an identifier other than in the global namespace could hide
undefined from that scope (although things can still be undefined)
• null is a word literal, so it's meaning can never be changed and attempting to do so will
throw an Error.

The similarities between null and undefined

null and undefined are both falsy.

if (null) console.log("won't be logged");
if (undefined) console.log("won't be logged");

Neither null or undefined equal false (see this question).

false == undefined // false

https://riptutorial.com/ 149