whatever.js

"use strict";

let getErrorRank = err => {
	let rankCounter = 0;
	let ranks = new Array(8).fill().map((item, i) => 1 / Math.pow(10, i)).slice(1);

	for (let i = 0; i < ranks.length; i++) {
		if (err < ranks[i]) {
			rankCounter++;
		} else {
			return rankCounter;
		}
	}
}

let getError = (value, error) => (value.initial > value.rounded ? value.initial - value.rounded : value.rounded - value.initial) + error;

let checkRoundedError = (value, error) => {
	let absoluteError = value * (error / 1e3);
	let errorRank = getErrorRank(absoluteError);
	let roundedValue = value.toFixed(errorRank - 1);
	let totalError = getError({ initial: value, rounded: roundedValue }, absoluteError);
	let totalErrorRank = getErrorRank(totalError);
	
	console.log(`Value: ${value}, Error: ${error}, Rounded: ${roundedValue}`);

	return totalErrorRank === errorRank;
}

[
	{ value: 7.521, error: 0.12 },
	{ value: 2.3544, error: 0.2 },
	{ value: 21.68563, error: 0.3 }
].map(num => console.log(`${checkRoundedError(num.value, num.error) ? '' : 'not'} all numbers are correct in a broad sense`));