paulirish

What forces layout / reflow

All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.

Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.

Element APIs

Getting box metrics

• elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent

Integralist

const flattenTco = ([first, ...rest], accumulator) => 
  (first === undefined)
    ? accumulator
    : (Array.isArray(first))
      ? flattenTco([...first, ...rest])
      : flattenTco(rest, accumulator.concat(first))
  
const flatten = (n) => flattenTco(n, []);
  
console.log(flatten([[1,[2,[[3]]]],4,[5,[[[6]]]]]))