JavaScript toFixed: Format a Number and Inspect Rounding Limits

The task

Prepare two display labels and inspect a value near a decimal rounding boundary. Keep formatting separate from later calculations.

Runnable Node 22 example

const assert = require('node:assert/strict');

const displayed = (7.5).toFixed(2);
assert.equal(displayed, '7.50');
assert.equal(typeof displayed, 'string');
assert.equal((2.55).toFixed(1), '2.5');
assert.equal(displayed + 1, '7.501');
assert.equal(Number(displayed) + 1, 8.5);
assert.throws(() => (1).toFixed(101), RangeError);
console.log(`formatted=${displayed}; boundary=${(2.55).toFixed(1)}`);

Expected output and explanation

The first result keeps two fractional digits, including a trailing zero. Adding one to that result performs string concatenation, which explains 7.501 rather than a numeric increment. Explicit conversion changes the operation but does not recover any precision previously lost.

The 2.55 example rounds as the actual stored Number dictates. It warns against assuming that every decimal-looking input is represented exactly. The digit-range assertion also demonstrates a rejected formatting request in Node22.

Pitfall to check

Do not use toFixed as evidence that a financial calculation is exact. Choose a numeric representation and rounding policy appropriate to that application. This fixture illustrates display formatting of ordinary measurements only.