数组遍历

[anjia]

• 遍历数组
  • for 最古老的循环，效率高。处理大量循环的时
  • for-of 只遍历对象上的，ES6
  • forEach 数组内部的方法。稀疏数组
  • for-in 会遍历继承链的对象属性。遍历对象的属性，仅可枚举类型
• 其它
  • map 产生新数组。数组的每项经过计算，产生的新值
  • filter 产生新数组。返回 true 的值
  • reduce 返回值。前项加后项，累计计算最终值
  • every 布尔值。遇到返回 false 的，就返回 false。 检测每一项是否符合条件
  • some 布尔值。遇到返回 true 的，就返回 true。 检查是否有符合条件的项

for > for-of > forEach > filter > map > for-in every > some

const arr = [1,2,3,4,5];
// const arr = [{'name': 'anjia', 'score': { 'math':90, 'eng':100}}, 5, 'test'];

function log(msg, prefix){
    prefix ? console.log('\n' + msg) : console.log(msg);
    
}

log('for', true);
for(let i=0, len=arr.length; i<len; i++){
    log(arr[i]);
}

log('for-of', true);
for(let item of arr){
    log(item);
}

log('forEach', true);
arr.forEach(item => {
    log(item);
});

log('for-in', true);
for(i in arr){  // 下标
    log(arr[i]);
}

function log(msg, prefix){
    prefix ? console.log('\n'+msg) : console.log(msg);
}

log('map', true);
var arr = [1,2,3,4,5];
var newArr = arr.map(item => {
    log(item);
    return item*2;
});
log(newArr);


log('filter', true);
var arr = [1,2,3,4,5,6];
let greater = arr.filter(item =>{
    log(item);
    return item > 3;
});
log(arr);
log(greater);


log('reduce', true);
let scores = [10, 20, 30];
let total = scores.reduce((sum, item)=>{
    console.log('sum=', sum, '  item=', item);
    return sum + item;
}, 0);
log(total);

[anjia]

e.g.

const members = ['anjia', 'luoqiang', 'lijianchun'];

// 移除'anjia'
const withoutAnjia = members.filter(item => item!=='anjia');
console.log(withoutAnjia === members);   // false
console.log(withoutAnjia, members);

// 把'anjia'替换成'zora'
const replaceAnjia = members.map(item => item==='anjia'?'zora':item);
console.log(replaceAnjia === members);  // false
console.log(replaceAnjia, members);

// 把所有人名大写
const strongMembers = members.map(item => item.toUpperCase());
console.log(strongMembers === members);   // false
console.log(strongMembers, members);

// 和另外一组进行合并
const otherMembers = ['peipei', 'yingying'];
const newMembers = [...members, ...otherMembers];
console.log(newMembers === members, newMembers === otherMembers); // false, false
console.log(newMembers, members);

• 创建了新对象，和原数组不是“对象引用”
• 写法更简洁，都是 immutable