Summary Notes on Truthy & Ternary Concepts in Javascript

A breakdown of Truthy & Ternary Concepts in Javascript.

Truthy & Falsy Evaluations

Taking the notion that when a variable is created, even without being assigned a value, it can still be evaluated. Most un-assigned values to variables are evaluted as false and thus can be said to be Falsy. See below list for examples. When a variable is assigned a value, (Not matching the exmaples from the list) the variable is evaluated to true. And it can be said that the variable is Truthy.

Example of Falsy evaluated values are as follows:

• 0
• Empty strings like "" or ''
• null which represents when there is no value at all
• undefined which represents when declared variable lacks a value.
• NaN, or Not a Number.

Example Code Block

let favoritePhrase = ''; // Evaluates to 'false'. Is Falsy

// The Else clause will trigger as the variable 'favouritePhrase' is falsy, and the conditional evaluates to false. 
// if (favoritePhrase) === if ('') === if (false)

if (favoritePhrase) {
  console.log("This string doesn't seem to be empty.");
} else {
  console.log('This string is definitely empty.');
}

Truthy and Falsy Assignment

You can write shorthand code for some forms of variable assignements, using logical operators, and Short-circuit evaluation. For example if you take the code block for username assignment to a variable.

let defaultName;
if (username) {
  defaultName = username;
} else {
  defaultName = 'Stranger';
}

Using logical operation as well as Truthy & Falsy assinment priciples whe can simplify this to a line.

let defaultName = username || 'Stranger';

Because || or statements check the left-hand condition first, the variable defaultName will be assigned the actual value of username if is truthy, and it will be assigned the value of ‘Stranger’ if username is falsy. This concept is also referred to as short-circuit evaluation.

Another Example Block

let tool = 'marker';

// Using short-circuit-evaluation to assign  writingUtensil variable below:
let writingUtensil = tool || 'pen'

console.log(`The ${writingUtensil} is mightier than the sword.`);

Ternary Operator

The ternary operator allows for a compact syntax in the case of binary (choosing between two choices) decisions. It accepts a condition followed by a ? operator, and then two expressions separated by a :. If the condition evaluates to truthy, the first expression is executed, otherwise, the second expression is executed.

let price = 10.5;
let day = "Monday";

day === "Monday" ? price -= 1.5 : price += 1.5;

Other Example code blocks

let isLocked = false;

isLocked ? console.log('You will need a key to open the door.'):
console.log('You will not need a key to open the door.');


let isCorrect = true;

isCorrect ? console.log('Correct!') :
console.log('Incorrect!');

let favoritePhrase = 'Love That!';

favoritePhrase === 'Love That!' ? console.log('I love that!') :
console.log("I don't love that!");