Falsy values

2 snippets across 2 stacks — JavaScript, Python

JSJavaScript

Falsy Values

syntax

false | 0 | -0 | 0n | "" | null | undefined | NaN

example

const values = [false, 0, -0, 0n, "", null, undefined, NaN];
const truthyOnes = values.filter(Boolean);
console.log(truthyOnes.length);

output

Note Everything else is truthy, including empty objects {}, empty arrays [], and the string "false". This trips up many developers.

PYPython

Truthiness Gotchas

PY · Common Mistakes

syntax

# Falsy: None, False, 0, 0.0, '', [], {}, set()

example

def process(data=None):
    # WRONG: if not data
    # This also catches empty list [], 0, and ""
    if not data:
        print("No data")  # Triggered by empty list too!

    # CORRECT: if data is None
    if data is None:
        print("Data is None")

process([])   # Might not intend to reject []
process(None)
process(0)

output

No data
No data
Data is None
No data

Note Empty collections, zero, empty string, and None are all falsy. When you specifically mean 'no value provided', check 'is None' rather than relying on truthiness.