JavaScript同步和异步面试题

Posted 2021-07-24 江州益彤

宏任务和微任务

js是单线程

宏任务

分类：setTimeout，setInterval，requrestAnimationFrame

微任务

分类：new Promise().then(回调)，process.nextTick
一、

async function async1() {
	console.log('async1 start');
	await async2();
	console.log('async1 end');
}
async function async2() {
	console.log('async2');
}

console.log('script start');

setTimeout(function() {
		console.log('setTimeout');
	},0)
	
async1();

new Promise(function(resolve) {
	console.log('promisel');
	resolve();
}).then(function() {
	console.log('promise2');
});

console.log('script end');

或（浏览器的原因，ie8以上的浏览器先输出then再执行其他）


二、

async function async1() {
	console.log('async1 start');
	await async2();
	console.log('async1 end');
}
async function async2() {
	console.log('async2');
}
console.log('script start');
setTimeout(function() {
	console.log( 'setTimeout');
}, 0)
async1(); 
console.log('script end');

script start
async1 start
async2
script end
async1 end
setTimeout
三、

function app() {
	setTimeout(() => {
		console.log("1-1");
		Promise.resolve().then(() => {
			console.log("2-1");
		});
	});
	
	console.log("1-2");
	Promise.resolve().then(() => {
		console.log("1-3");
		setTimeout(() => {
			console.log("3-1");
		});
	});
}
app();