src/extent.js
import { getSize as getExtentSize } from 'ol/extent'
/**
* returns an extent with the same center but width and height miltiplied by the given value
* @param {ol.Extent} extent
* @param {number} value
* @returns {ol.Extent}
*/
export function multiplyExtent (extent, value) {
const size = getExtentSize(extent)
const addX = size[0] * (value - 1) / 2
const addY = size[1] * (value - 1) / 2
return [extent[0] - addX, extent[1] - addY, extent[2] + addX, extent[3] + addY]
}