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javascript-falsy-y-truthy

Falsy and Truthy Values in TypeScript

  • 3 min
  • Beginner

In JavaScript, values are divided into two categories: truthy and falsy. These terms refer to how values behave when evaluated as a boolean.

  • truthy values evaluate as true values
  • falsy values evaluate as false values

This automatic conversion happens when the value is used in a context that expects a boolean (for example, when used as a condition in a conditional statement, or compared with an operator).

truthy and falsy are two concepts used very frequently in JavaScript. But don’t worry, they are very easy to understand.

If the words seem a bit absurd to you, don’t worry… you’re not the only one 😄

Falsy Values

falsy values are those that, when evaluated in a boolean context, are considered as false.

There are only six values in JavaScript that are considered falsy:

  1. false: The boolean value false is obviously falsy.
  2. 0: The number zero is falsy.
  3. "" (empty string): A string without characters is falsy.
  4. null: Represents the absence of any value and is falsy.
  5. undefined: Means a variable has been declared but not assigned, and is falsy.
  6. NaN: Means “Not-a-Number” which is falsy (it’s the result of an invalid mathematical operation)

Let’s see it with an example,

if (!0) {
    console.log("0 is a falsy value");
}

if (!null) {
    console.log("null is a falsy value");
}
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Truthy Values

truthy values are those that, when evaluated in a boolean context, are considered as true.

Any value that is not falsy is considered truthy. This includes practically everything else:

  • Numbers other than 0: All numbers different from 0, whether positive or negative
  • Non-empty strings: Any string with at least one character, including “0” or “false”
  • Objects and arrays: Any object or array, even empty ones
  • The boolean value true: Obviously 😉

For example,

if ("hello") {
    console.log("A non-empty string is truthy");
}

if ({}) {
    console.log("An empty object is truthy");
}
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Importance of Truthy and Falsy

In JavaScript (and in other languages to a lesser extent), some programmers have the habit of relying on the implicit evaluation of these properties to “simplify” the code.

This way they save themselves from explicitly comparing with true or false (maybe they get charged per letter or something)

For example:

let username = "John";

if (username) {
    console.log("The user has provided a name");
} else {
    console.log("The username is empty");
}
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In this code,

  • username is evaluated directly in the if condition.
  • If username is a non-empty string (truthy), the first branch will execute.
  • If it is an empty string (falsy), the second branch will execute.

Let me say this… don’t do that. Don’t rely on automatic conversions to booleans.

If you want to perform a check, do it correctly. Because later someone changes something in the code, and your conditional starts working incorrectly.

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