Built-in Functions

In short: JavaScript ships with a toolbox of functions available in every file, with no import needed — Math.round, Math.max, parseInt, isNaN, Number.isInteger, and many more. They cover rounding, comparing, random numbers, and safely checking a value's shape. Most behave exactly as the name suggests, but a few have a sharp edge worth memorising, because they don't fail loudly when misused.

Rounding and comparing without writing your own logic

Math bundles the everyday numeric helpers:

Math.round rounds to the nearest whole number, Math.max/Math.min take any number of arguments and return the largest/smallest. No loop, no manual comparison — the toolbox already has these.

Reading numbers out of messy text

parseInt and parseFloat, from the type-casting lessons, live here too — global functions, not tied to any object:

parseInt/parseFloat read the leading numeric part and stop at the first character that doesn't fit. isNaN checks whether a value, after being coerced to a number, comes out as NaN — "abc" * 2 already is NaN, so this is true. Number.isInteger checks for a genuine whole number, no coercion involved.

Two traps: empty Math.max(), and isNaN's coercion

Both functions have a documented, surprising edge case:

Math.max() with nothing to compare returns -Infinity (anything beats "nothing"), and Math.min() returns Infinity for the same reason — surprising if you expected 0 or an error. isNaN("hello") is true because the global version coerces "hello" to a number first — which is NaN — before checking. Number.isNaN("hello") does no coercion at all, so a string is never NaN itself, and it correctly says false. Prefer Number.isNaN when you want to know "is this value literally NaN," not "would this coerce to NaN."

Quick practice