function greet(name) {
 return `Hello, ${name}!`;
}

greet('Cristina'); // => 'Hello, Cristina!'

const persons = ['Cristina', 'Ana'];

const messages = persons.map(greet);
messages; // => ['Hello, Cristina!', 'Hello, Ana!'] 

function map(array, callback) {
 const mappedArray = [];
 for (const item of array) { 
 mappedArray.push(
 callback(item) );
 }
 return mappedArray;
}

function greet(name) {
 return `Hello, ${name}!`;
}

const persons = ['Cristina', 'Ana'];

const messages = map(persons, greet);messages; // => ['Hello, Cristina!', 'Hello, Ana!'] 

function map(array, callback) {
 console.log('map() starts');
 const mappedArray = [];
 for (const item of array) { mappedArray.push(callback(item)) }
 console.log('map() completed');
 return mappedArray;
}

function greet(name) {
 console.log('greet() called');
 return `Hello, ${name}!`;
}

const persons = ['Cristina'];

map(persons, greet);
// logs 'map() starts'
// logs 'greet() called'
// logs 'map() completed'

// Examples of synchronous callbacks on arrays
const persons = ['Ana', 'Elena'];

persons.forEach(
 function callback(name) { console.log(name);
 }
);
// logs 'Ana'
// logs 'Elena'

const nameStartingA = persons.find(
 function callback(name) { return name[0].toLowerCase() === 'a';
 }
);
nameStartingA; // => 'Ana'

const countStartingA = persons.reduce(
 function callback(count, name) { const startsA = name[0].toLowerCase() === 'a';
 return startsA "htmlcode">

// Examples of synchronous callbacks on strings
const person = 'Cristina';

// Replace 'i' with '1'
person.replace(/./g, 
 function(char) { return char.toLowerCase() === 'i' "htmlcode">

console.log('setTimeout() starts');
setTimeout(function later() {
 console.log('later() called');
}, 2000);
console.log('setTimeout() completed');

// logs 'setTimeout() starts'
// logs 'setTimeout() completed'
// logs 'later() called' (after 2 seconds)




later() 是一个异步回调，因为 setTimeout(later，2000) 启动并完成了执行，但是 later() 在 2 秒后执行。
异步调用回调的步骤：

  
高阶函数开始执行：'setTimeout()starts'
  
高阶函数完成其执行：'setTimeout() completed'
  
回调函数在 2 秒钟后执行：'later() called'

  

异步回调的例子
计时器函数异步调用回调：


setTimeout(function later() {
 console.log('2 seconds have passed!');
}, 2000);
// After 2 seconds logs '2 seconds have passed!'

setInterval(function repeat() {
 console.log('Every 2 seconds');
}, 2000);
// Each 2 seconds logs 'Every 2 seconds!' 




DOM 事件侦听器还异步调用事件处理函数（回调函数的子类型）：


const myButton = document.getElementById('myButton');

myButton.addEventListener('click', function handler() {
 console.log('Button clicked!');
});
// Logs 'Button clicked!' when the button is clicked




4.异步回调函数与异步函数


在函数定义之前加上特殊关键字 async 会创建一个异步函数：


async function fetchUserNames() {
 const resp = await fetch('https://api.github.com/users"htmlcode">

const button = document.getElementById('fetchUsersButton');

button.addEventListener('click', fetchUserNames);




总结


回调是一个可以作为参数传给另一个函数（高阶函数）执行的函数。
回调函数有两种：同步和异步。
同步回调是阻塞的。
异步回调是非阻塞的。
最后考考你：setTimeout(callback，0) 执行 callback 时是同步还是异步的？

                                
                                

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