let list = [1, 2, 3, 4, 5];
let other = list.map((d, i) => {
  return d * 2;
});
console.log(other);
// print: [2, 4, 6, 8, 10]

let list = [1, 2, 3, 4, 5];
let other = list.filter((d, i) => {
  return d % 2;
});
console.log(other);
// print: [1, 3, 5]

let list = [1, 2, 3, 4, 5];
list.some((d, i) => {
  console.log(d, i);
  return d > 3;
});
// print: 1,0 2,1 3,2 4,3
// return false

let list = [1, 2, 3, 4, 5];
list.every((d, i) => {
  console.log(d, i);
  return d < 3;
});
// print: 1,0 2,1 3,2
// return false

let list = [1, 2, 3, 4, 5];
let other = [];
list.forEach((d, i) => {
  other.push(d * 2);
});
console.log(other);
// print: [2, 4, 6, 8, 10]

var obj = {a:1, b:2, c:3};
for (var prop in obj) {
 console.log("obj." + prop + " = " + obj[prop]);
}
// print: "obj.a = 1" "obj.b = 2" "obj.c = 3"

// 字符串
let str = "hello";
for (let s of str) {
 console.log(s); // h e l l o
}
// 遍历数组
let list = [1, 2, 3, 4, 5];
for (let e of list) {
  console.log(e);
}
// print: 1 2 3 4 5
// 遍历对象
obj = {a:1, b:2, c:3};
for (let key of Object.keys(obj)) {
 console.log(key, obj[key]);
}
// print: a 1 b 2 c 3

// entries
let arr = ['a', 'b', 'c'];
for (let pair of arr.entries()) {
 console.log(pair);
}
// [0, 'a']
// [1, 'b']
// [2, 'c']