arithmetic.js

function arithmetic (input) {
    let expression = input.replace(/\s/g, '');

    const numbers = /((?:\d|\.)+)/;
    const groups = /\(([^(]+?)\)/;
    const methods = [
        { op: /(\^|\*{2})/, fn: (x, o, y) => x ** y },
        { op: /(\*|\/|\%)/, fn: (x, o, y) => {
            if (o === '*') return x * y;
            if (o === '/') return x / y;
            if (o === '%') return x % y;
        } },
        { op: /(\+|\-)/, fn: (x, o, y) => {
            if (o === '+') return x + y;
            if (o === '-') return x - y;
        } }
    ];

    const evaluate = terms => {

        for (const { op, fn } of methods) {
            const regex = new RegExp(numbers.source + op.source + numbers.source);

            while (terms.match(regex)) {
                const [ term, x, o, y ] = terms.match(regex);
                terms = terms.replace(term, fn(Number(x), o, Number(y)));
            }
        }

        return terms;
    }

    const negative = new RegExp(`(?:(^)|${ methods.map(m => m.op.source).join('|') })\-${ numbers.source }`);

    while (expression.match(negative)) {
        const [ term ] = expression.match(negative);
        expression = expression.replace(term, `(0${ term })`);
    }

    while (expression.match(groups)) {
        const [ group, terms ] = expression.match(groups);
        expression = expression.replace(group, evaluate(terms));
    }

    return Number(evaluate(expression));
}