JavaScript Christmas

Truthy and falsy

A 1 minute read written by
Jan Amundsen

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What is the truth?

const value = new Boolean(false);
if (value) {
  // code

If we consider the code snippet above, what does the condition evaluate to? It might, or might not surprise you that the condition evaluate to true. But why?

Everybody that know JavaScript is familiar with the concept of true and false, the primitive boolean values. Truthy and falsy are used to evaluate a condition on values other than the primitive boolean values. This help us evaluate conditions without first performing an explicit type convertion on a value to a primitive boolean type. In other words, we can write:

const user = { name: "Jan Karlsen" }
if ( {
  // code

instead of:

if ( !== null && !== undefined && !== "") {
  // code

What values are truthy and what values are falsy? The easiest way for me to remember, is that all values are truthy, unless defined as falsy. The falsy values are as follows: false, 0, "", null, undefined, and NaN.

So, the reason the condition in the first code snippet evaluated to true, is that the value used is a Boolean object, and a object is truthy in JavaScript. Even an empty object {} is truthy, as we can see from the list of falsy values above.

All the expressions below will evaluate to true:

if (true)
if ({})
if (-1)
if ("example")
if ([])
if (() => {})
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