• Explain What a Boolean Expression Is
• Describe Truthy and Falsey Values in JavaScript

Quite commonly in our computer programs, we want to execute code conditionally, i.e., if some condition is true (or false). For example:

• If the user has entered the correct login credentials, then log them into our web app
• If there isn't enough money in the user's account to execute the transfer, then send them an error message

In the following lessons, we will learn about operators we can use to construct those conditional statements. Before we get to that, however, we need to understand boolean expressions and the concepts of truthy-ness and falsey-ness.

We have learned that expressions are code statements that return a value: the constant expression returns the value of the constant, the assignment expression returns the value that is assigned, and the variable lookup expression returns the value of a variable. We have also learned that JavaScript has a Boolean data type that has only two values: true or false. As you might expect, therefore, a Boolean expression is an expression that returns either true or false.

All values in Javascript, of any data type (String, Number, etc.), are treated as either truthy or falsey. The following values are falsey:

• false
• null
• undefined
• 0
• NaN
• An empty string ("", '')

Every other value is truthy.

To check whether a value is truthy or falsey, we can pass it to the global Boolean object, which converts the value into its boolean equivalent. You may want to try these out yourself in replit.

Boolean(false);
// => false

Boolean(null);
// => false

Boolean(undefined);
// => false

Boolean(0);
// => false

Boolean(NaN);
// => false

Boolean("");
// => false

Boolean(true);
// => true

Boolean(42);
// => true

Boolean("Hello, world!");
// => true

Don't worry if this seems a little theoretical at this point. We will learn how this information is used in constructing Boolean expressions in the following lessons.

In this lesson, we've learned what a Boolean expression is. We've also learned which values in JavaScript are considered truthy and which are considered falsey. With these understandings, we are now ready to learn how to construct Boolean expressions that return true or false.

• MDN
  • Truthy and falsey