Truthy and Falsy

TypeScript considers some values as "falsy". When evaluated inside a boolean expression, they will act as false. Here are a few examples of falsy variables in TypeScript:

• false
• 0
• NaN (the "not a number" value)
• "" (empty string)
• [] (empty array)
• null
• undefined

All other values will be considered "truthy", meaning that when evaluated as a boolean expression, they will act as true.

The || (logical OR) operator can be used to select the first non-falsy value. For example:

const bananas = 3;
const apples = 0;

// will be set to 3 since bananas is truthy
const numberOfBananasOrApples = bananas || apples;

const oranges = 0;
const pears = 4;

// will be set to 4 since oranges is falsy
const numberOfBananasOrApples = oranges || pears;

Using the double not operator combination (!!)

To force a variable into it's boolean representation from either truthy or falsy to either true or false, use !! as follows:

For example:

const bananas = 3;
const apples = 0;

// will be set to true because bananas is greater than zero
const atLeastOne = !!bananas || !!apples;

The logical not operator forces TypeScript to turn the variable into a boolean value, and using it twice will flip the condition to true if it is truthy and false otherwise.

The nullish coalescing operator (??)

TypeScript allows to return a "fallback" value if the left side of the operator is either null or undefined. It is similar to using the logical OR operator || but will choose the right value only if the left side is either null or undefined, whereas the || operator will choose the left side only if the value is truthy.

const apples = 0;
const oranges = 3;

// will be set to 0 since apples is a number
const applesOrOranges = apples ?? oranges;

// will be set to 3 since apples is falsy
const numberOfApplesOrOranges = apples || oranges;

Exercise

Select the first number from the variables a, b, and c which is not null or undefined.