You use a for..in loop for this. Be sure to check if the object owns the properties or all inherited properties are shown as well. An example is like this:

var obj = {a: 1, b: 2};
for (var key in obj) {
  if (obj.hasOwnProperty(key)) {
    var val = obj[key];
    console.log(val);
  }
}

Or if you need recursion to walk through all the properties:

var obj = {a: 1, b: 2, c: {a: 1, b: 2}};
function walk(obj) {
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
      var val = obj[key];
      console.log(val);
      walk(val);
    }
  }
}
walk(obj);

var arr = [ {"id":"10", "class": "child-of-9"}, {"id":"11", "class": "child-of-10"}];
    
for (var i = 0; i < arr.length; i++){
  document.write("<br><br>array index: " + i);
  var obj = arr[i];
  for (var key in obj){
    var value = obj[key];
    document.write("<br> - " + key + ": " + value);
  }
}

note: the for-in method is cool for simple objects. Not very smart to use with DOM object.

Depends how many nested levels there are. If it's as the example shows, then a simple loop will get you what you want. If you need the access the key of the object and the objects nested in the arrays use for in. If not, you can use for of.

for (const key in obj) {
    // key here would be "asdf" in your example
    for (const index in obj[key]) {
        // index here would be the index of the array of "asdf"
        for (const nestedObjKey in obj[key][index]) {
            // nestedObjKey would be "date", etc. in your example
            // obj[key][index][nestedObjKey] would be "2020-1-22", etc. in your example
        }
    }
}