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let a = [1, 2, 3, null, 5];
a.six = 6;
a.seven = 7;

for (let i = 0; i < a.length; i++) {
    console.log(a[i]); // 1, 2, 3, null, 5
}

for (let v of a) {
    console.log(v); // 1, 2, 3, null, 5
}

for (let k in a) {
    console.log(a[k]); // 1, 2, 3, null, 5, 6, 7
}