[Nexus]

(() => {
    const arrayNumbers = [-5, 0, 5, 10, 15, 20, 25, 30, 35];
    const n = 23.45
    const res = arrayNumbers[arrayNumbers.reduce(function (res, value, index) {
        return (value = Math.abs(n - value)) < res.value ? {index, value} : res;
    }, {index: null, value: Infinity}).index];

    alert(res);
})();

upd.

(() => {
    const n = prompt('Enter the number', 11.38) || 23.45;
    const arrayNumbers = [-5, 0, 5, 10, 15, 20, 25, 30, 35];
    const res = arrayNumbers[arrayNumbers.reduce(function (res, distance, index) {
        return (distance = Math.abs(n - distance)) < res.distance ? {index, distance} : res;
    }, {index: 0, distance: Infinity}).index];

    alert(`n = ${n}; res = ${res}`);
})();