for ([initialExpression]; [condition]; [incrementExpression]) 
    statement(s)

let sum = 0;
for (let i = 1; i <= 50; i++) {
   sum = sum + i;
}
console.log("Sum = " + sum);    // => Sum = 1275

let i = 0;
while (i < 3) {
    console.log(i);
    i++;
}
// 输出: 0 1 2

let j = 0;
do {
    console.log(j);
    j++;
} while (j < 3);
// 输出: 0 1 2

const person = { name: "Alice", age: 25 };
for (let key in person) {
    console.log(key, ":", person[key]);
}
// 输出:
// name : Alice
// age : 25