main difference between forEach and for loop

main difference between forEach and for loop

the main difference you must be aware about array iteration is that forEach iterate only non-empty slots, while for iterate using a custom index value, so if you start at zero and end at length-1 iterate all slots... for..in act as forEach (iterate over non-empty slots) for..of act as usual for iteration (all slots values) what could be unobvious is concept of empty slots, wich when accessed from index return undefined as value, as do an non-empty slot assigned with undefined ;P

given this code initialization:

let a = Array(5);

a[2] = 'item';
a[3] = undefined;

by clicking test codes button below, output is logged at console: in sololearn app' the console pop up automatically (and you must manually expand/scroll to see last results), but in sololearn website you must open the browser console to see the result ^^

'forEach' code:

console.log('forEach loop:\n');

a.forEach((v,i) => {
    console.log(i+' '+v);
});

output:

forEach loop:

2 item
3 undefined

'for' code:

console.log('for loop:\n');

for (let i=0; i<a.length; ++i) {
    console.log(i+' '+a[i]);
}

output:

for loop:

0 undefined
1 undefined
2 item
3 undefined
4 undefined

'for..in' code:

console.log('for..in loop:\n');

for (let i in a) {
    console.log(i+' '+a[i]);
}

output:

for..in loop:

2 item
3 undefined

'for..of' code:

console.log('for..of loop:\n');

for (let v of a) {
    console.log(v);
}

output:

for..of loop:

undefined
undefined
item
undefined
undefined