map 与 for 循环的区别

map 与 for 循环的区别：

Talk is cheap. Show me the code.

第 1 组：
1️⃣

const arr1 = [1, 2, 3];
const arr2 = [];
for(let i = 0; i < arr1.length; i++) {
  arr2.push(arr1[i] * 2);
}
console.log(arr2); // [ 2, 4, 6 ]

2️⃣

const arr1 = [1, 2, 3];
const arr2 = arr1.map(item => item * 2);
console.log(arr2);  // [ 2, 4, 6 ]

第 2 组：
1️⃣

const birthYear = [1975, 1997, 2002, 1995, 1985];
const ages = [];
for(let i = 0; i < birthYear.length; i++) {
  let age = 2018 - birthYear[i];
  ages.push(age);
}
console.log(ages); // [ 43, 21, 16, 23, 33 ]

2️⃣

const birthYear = [1975, 1997, 2002, 1995, 1985];
const ages = birthYear.map(year => 2018 - year);
console.log(ages); // [ 43, 21, 16, 23, 33 ]

第 3 组：
1️⃣

const persons = [
  { name: 'Peter', age: 16 },
  { name: 'Mark', age: 18 },
  { name: 'John', age: 27 },
  { name: 'Jane', age: 14 },
  { name: 'Tony', age: 24},
];
const fullAge = [];
for(let i = 0; i < persons.length; i++) {
  if(persons[i].age >= 18) {
    fullAge.push(persons[i]);
  }
}
console.log(fullAge);

2️⃣

const persons = [
  { name: 'Peter', age: 16 },
  { name: 'Mark', age: 18 },
  { name: 'John', age: 27 },
  { name: 'Jane', age: 14 },
  { name: 'Tony', age: 24},
];
const fullAge = persons.filter(person => person.age >= 18);
console.log(fullAge);

第 4 组：
1️⃣

const arr = [5, 7, 1, 8, 4];
let sum = 0;
for(let i = 0; i < arr.length; i++) {
  sum = sum + arr[i];
}
console.log(sum); // 25

2️⃣

const arr = [5, 7, 1, 8, 4];
const sum = arr.reduce(function(accumulator, currentValue) {
  return accumulator + currentValue;
});
console.log(sum); // 25

参考原文