Check if a character is a digit

Test that a string is exactly one ASCII digit from 0 to 9.

JavaScript Version 1.0.0 · MIT · .md

function isDigit(char) { return /^[0-9]$/.test(char); }

Input & expected output

"7"

true

The edge case

" "

false

Know the limits

• The original `char < 10` accepted "", " " and "-1" because of numeric coercion, and one variant referenced an undefined variable. This version tests exactly one ASCII digit.
• Other Unicode digits, such as Arabic-Indic numerals, return false.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Check whether all values are equal — Compare every item with the first using Object.is.
• Check whether an object is empty — Check for own enumerable string keys on a JSON object.
• Check whether an integer is even — Test parity for safe integers, including zero and negative values.