for

for(let i = 0; i < 3; i++)
  output.innerText += (i + "\n");
  

- loops over index

for in

var obj1 = {
  "key1": "value1", 
  "key2": "value2"
};
for(let k in obj1)
  output.innerText += (k + '\n');
  

- loops over keys of objects;

- in case of arrays, their numeric index.

for of

var arr1= ["element1", "element2"];
for(let el of arr1)
  output.innerText += (el + '\n');
  

- loops over values of objects;

- in case of arrays, their elements.

forEach

var arr2 = ["value3", "value4"];

function myFunc(element,index){
  output.innerText += `${index} -> ${element}\n`;
}

arr2.forEach(myFunc);
  

- forEach is an Array prototypical method, accepting a function as argument.

- The function can have up to three parameters, which are the element, the index and the array itself respectively.