True False

lesson_2.md

Truthy and Falsey

Intro

In JavaScript things can either be true or false. Often we want to perform some code based on weather a condition is true or false.

	// Pretend code
	if TRUE do THIS
	otherwise do THAT

Let's look at common truthy values:

• The Boolean true - true
• A non empty String - 'hi', 'a'
• A non zero Number - -4, 50, 10045, 1
• Any Array - [0, 1, 2], ['a'], []
• Any Object - {}, { name: 'bobby' }

Let's look at common falsey values:

• The Boolean false - false
• The number 0 - 0
• An empty String - ''
• NaN
• null
• undefined

Testing to see if something is true or false

Often in your code you need to test multiple things and act on them based on the following:

• At least one of the things is true/false
• If ALL of the things are true/false

To do this, we use AND and OR logic gates:

AND logic gate

A	B	RESULT
0	0	FALSE
1	0	FALSE
0	1	FALSE
1	1	TRUE

OR logic gate

A	B	RESULT
0	0	FALSE
1	0	TRUE
0	1	TRUE
1	1	TRUE

It is possible to use as many parameters as you want to test, but it starts to get very difficult to follow after 3.

Reversing True and False

A special symbol in JavaScript and most other programming languages is the NOT symbol. This is the exclamation mark !

NOT logic gate

A	RESULT
0	TRUE
1	FALSE

Convert anything to a boolean

You can take anything and convert it to a simple true false boolean value by using the special JavaScript symbol !! (two exclamation marks).

Running this on the number 42: !!42 will become the Boolean true Running this on the number 0: !!0 will become the Boolean false