These three versions of equivalent code demonstrates how code should be self-explainable

empatheticCode.js

dataArray.reduce((a, b) => [...a,
  ...(b.data && b.data.isImportant && b.data.value ? [b.data.value] : [])
], []);

/**
 * Thwe horrific JavaScript snippet above is equivalent to the following
 */

dataArray
  .filter((x) => x.data && x.data.isImportant && x.data.value)
  .map((x) => x.data.value);

/**
 * And if you really insist on a single iteration, meaningful names
 * improve readability significantly
 */

dataArray.reduce((result, element) => {
  if (element.data) {
    const { isImportant, value } = element.data;
    if (isImportant && value) return [...result, value];
  }
  return result;
}, []);