Here you go:

Object.assign({}, ...function _flatten(o) { return [].concat(...Object.keys(o).map(k => typeof o[k] === 'object' ? _flatten(o[k]) : ({[k]: o[k]})))}(yourObject))

Summary: recursively create an array of one-property objects, then combine them all with Object.assign.

This uses ES6 features including Object.assign or the spread operator, but it should be easy enough to rewrite not to require them.

For those who don't care about the one-line craziness and would prefer to be able to actually read it (depending on your definition of readability):

Object.assign(
  {}, 
  ...function _flatten(o) { 
    return [].concat(...Object.keys(o)
      .map(k => 
        typeof o[k] === 'object' ?
          _flatten(o[k]) : 
          ({[k]: o[k]})
      )
    );
  }(yourObject)
)

You can try to do it with Array.reduce. It is better than original code because of 2 points:

• We don't need to create an additional variable to show answer (newArr in my example is just for clear code, we can also not to use it and console.log the result directly)
• In original code we use array.map method to save the "mutated" array in another variable and then destructing it so we have 2 loops. But we can do it with 1 loop, we just need to get each item in array and put it to another array without saving it anywhere

So I think that my example is less complex and faster.

P.S. I also think that we don't need pretty variable.

const content = [
    { role: 'row', id: 0, expressions: { formula: [ { id: 'L34', name: 'CommodityName' }, { id: 'L4', name: 'ItemId' } ] } },
    { role: 'column', id: 1, expressions: { formula: [ { id:'LC', name: 'Year' }, { id: 'LA', name: 'Month' } ] } },
    { role: 'body',  id: 2, expressions: { formula: [ { id:'L1B', name: 'Demand' } ] } }
];


const newArr = content.reduce((formulas, contentItem) => {
  contentItem.expressions.formula.forEach(formulaItem => formulas.push({
    contentId: contentItem.id,
    role: contentItem.role,
    ...formulaItem
  }));
  return formulas;
}, [])

console.log(newArr);