Python vs JavaScript: Truthy and Falsy values

One of the lesser-known concepts in both Python and JavaScript is truthy and falsy values. This means that, in both languages, there is a certain set of values that evaluate to FALSE and the remaining values evaluate to TRUE.

Python

Falsy values are:

• None

• False

• Numbers equal to zero: 0, 0.0, 0j

• Empty sequences and collections. Common ones are: [], (), '', range(0), set(), {} , empty bytearray, empty memoryview

• Any object for which obj.__bool__() returns False. Or obj.__len__() returns 0 if obj.__bool__ is undefined.

if []:
    print("Empty list is True")
else:
    print("Empty list is False")

# Output: Empty list is False

All remaining values evaluate to true. This means that the following code prints true:

print(bool("[]"))  # Output: True

JavaScript

Falsy values are null, undefined, NaN, '', 0, false

As you can see, the number of falsy values is more in Python as compared to JavaScript.

if ('') {
    console.log('Empty string is true')
}
else {
    console.log('Empty string is false')
}

// Output: Empty string is false

All remaining values evaluate to true. This means that the following code prints true:

console.log(Boolean("undefined"))  // Output: true

Code example

You can use this concept to shorten the code you write. For example, let's say you want to find all nonempty strings in a list of strings that you got from a database

let names = ["Aaron", "James", "", "Julie", null, "Jane"] 

console.log(names.filter(name => name).join(','));

names = ["Aaron", "James", "", "Julie", None, "Jane"] 

print(','.join(name for name in names if name))

Output in both cases: Aaron, James, Julie, Jane

Summary

We saw the values which are falsy in both JS and Python and also one of their application in code. Let's summarize all the falsy values in a table: