Javascript multiple value object to simple array

forEach is available on arrays, but during the first iteration obj1[key] is effectively obj1.A, which returns an object {bottom: 63, mid: 4, top: 15}, which doesn't have forEach, hence the error.

This might be a good use of reduce:

obj1 = {A: {bottom: 63, mid: 4, top: 15}, B: {bottom: 30, mid: 23, top: 5}, C: {bottom: 41, mid: 25, top: 16}}

const result = Object.entries(obj1).reduce(
  // group is the key from obj1, e.g. "A"
  // properties is the object for that key, e.g. { bottom: 63, mid: 4, top: 15 }
  (acc, [group, properties]) => [
    ...acc, { group, ...properties }
  ], []
);

console.log(result);

obj1[key] will return an object like {bottom: 63, mid: 4, top: 15}, which doesnt have a forEach function. The simplest way to do what you are trying to do would be to add the group key to the object each loop and push it to results:

// for each key in obj
Object.keys(obj1).forEach(key => {
    obj1[key].group = key;
    result.push(obj1[key]);
})

Currently, you are trying to iterate through an object obj1[key].forEach which does not have a forEach function. the function belongs to Array prototype.

Try this instead:

obj1 ={A: {bottom: 63, mid: 4, top: 15}, B: {bottom: 30, mid: 23, top: 5}, C: {bottom: 41, mid: 25, top: 16}}

const result = []

// for each key in obj
Object.keys(obj1).forEach(key => {
    // for each array element of the property obj[key]
    Object.keys(obj1[key]).forEach((value, index) => {// in this line I'm getting the error
        // if an object doesn't exists at the current index in result
        // create it
        if (!result[index]) {
            result[index] = {}
        }
        // at the result index, set the key to the current value
        result[index][key] = value
    })
})
console.log(result)

you can use a map function base on obj1's keys

    var obj1 = {A: {bottom: 63, mid: 4, top: 15}, B: {bottom: 30, mid: 23, top: 5}, C: {bottom: 41, mid: 25, top: 16}};

    var obj2 = Object.keys(obj1).map(function(k) {
      var clone = JSON.parse(JSON.stringify(obj1[k])); // use lobash for cloning
      clone.group = k;
      return clone;
    });

    console.log(obj2)

Iterating over the keys using Object.keys and followed by using .map to form the required result

var obj1 = {
  A: { bottom: 63, mid: 4, top: 15 },
  B: { bottom: 30, mid: 23, top: 5 },
  C: { bottom: 41, mid: 25, top: 16 }
};

var out = Object.keys(obj1).map(i => {
  return {
    group: i,
    ...obj1[i]
  };
});

console.log(out);