Coercion - Truthy and Fasly Values In JavaScript

As JavaScript developers, we are very familiar with the Boolean data type. This data type can assume two values: true and false.

When a value is not exactly a boolean type (true or false) and you try to perform a condition with it, what does JavaScript do? Take a look at this code:

const name = "Mad Max";
if(name) {
  console.log("Executed");
}
// Output: "Executed"

It might be surprising that the if statement was executed even though the value in the variable name is not a boolean. What is happening here is called Coercion. JavaScript tries to coerce (partially convert, but does not alert the value) any value that is not a boolean to a truthy / falsy value so that it can perform a given condition, like in the example above.

If a value is truthy, it is taken as the boolean true, otherwise it's falsy, so it is taken as false. There are certain rules to determine if a value is truthy or falsy:

Rules For Truthy Values:

1. Any number (including negative numbers) except 0

2. All non-empty string.

3. All empty and non-empty array or objects.

Rules For Fasly Values:

1. Number zero (0)

2. Empty string.

3. null, undefined, NaN.

Using this rules, we can tell before-hand if a value is truthy or fasly. Knowledge on truthy and falsy values can help you write shorter codes, like in this example:

const username = "";

// No understanding of coercion / truthy and fasly values
if(username === "") {
  console.log("Username is empty");
}

// Understaing of coercion / truthy and fasly values
if(!username) {
  console.log("Username is empty");
}

The code works the same, but you have a much smaller code in the second if statement. The username is assigned to an empty string, by using the rules above, it is a fasly value. By using the negation sign on the fasly value we get a truthy value, which execute the if statement. On the other hand, if the username was a non-empty string, it will be a truthy value, if we then add the negation sign, it will be a falsy value which means that the if statment won't execute. Another example:

const isLoggedIn;

if(isLoggedIn) {
  console.log("User logged in");
} else {
  console.log("User not logged in");
}
// Output: "User not logged in"

This is because by default any time you do not initialize a variable in JavaScript, it is undefined. And by using the rules above, undefined is a fasly value, therefore the else block runs.

END.