JavaScript Control Flow Truthy and Falsy

Learning objective: By the end of this lesson, students will understand the concept of truthy and falsy values in JavaScript.

Truthy and Falsy

JavaScript has the primitive boolean values of true and false, but it also has the idea of truthy and falsy. So what are truthy and falsy, and why don’t we use true and false for everything?

Understanding Truthy and Falsy

Truthy and falsy are conceptual and attempt to treat non-boolean expressions as booleans (true or false) during runtime. This concept is especially useful in conditions like if statements, where you might want to check if a variable exists or has a meaningful value without explicitly comparing it to true or false.

What is Falsy?

Most things in JavaScript are truthy, so it’s easier to remember what is falsy.

These two data types are always falsy:

• null: Represents the intentional absence of any object value.
• undefined: Indicates that a variable has not been assigned a value.

These four values are also falsy:

• false: The boolean value of false.
• 0 (zero): The number zero.
• NaN (Not a Number): Indicates a value that is not a valid number.
• '' (An empty string): Strings are falsy when they are empty. Note, a string with a value of a space (' ') is not empty and therefore is truthy.

What is Truthy?

Everything else is truthy! This means most objects, including arrays and functions, non-zero numbers, and non-empty strings, are considered true in a boolean context.

Practical Use

The concept of truthiness and falsyness in JavaScript is not just a theoretical idea, but it has very practical applications in coding, particularly in making your code more concise and readable.

Simplifying Conditions

Truthy and falsy values allow us to write conditions that directly check the value itself, rather than writing out a full comparison. This makes the code cleaner and often easier to understand at a glance.

Be Cautious

However, it’s essential to be cautious when using this feature. Misunderstanding the truthiness or falsiness of a value can lead to bugs, especially if you’re not fully aware of the potential values a variable can hold.

Example in Code

Consider the following example:

let string = "truthy value";

// Here, 'if (string)' is checking if 'string' is not an empty string, null, undefined, false, 0, or NaN.
if (string) {
  // This block of code will run because 'string' contains a non-falsy value.
}

The above syntax is preferable to the following equivalent statement:

if (string !== "") {
  // this is a more lengthy way of trying to make the same check without 
  // relying on the inherent nature of truthy/falsy values in JavaScript
}