nodejs 中实现异步通常有标题中三种方法,但是如果不理解它们在node event loop中的生命周期的具体执行位置,很容易遇到一些莫民奇妙的困惑.
比如:
let bar;
function asyncApi(callback) {
callback()
}
asyncApi(value => {
console.log(value + "bar", bar);
});
bar = 1;
what will happen?
😆 因为其实是同步方法,所以
undefinedbar undefined
言归正传 if ... then ?
const fs = require("fs");
let bar;
function someAsyncApiCall(callback) {
setImmediate(callback, "immediate");
setTimeout(()=>{callback('setTimeout')}, 0)
}
someAsyncApiCall(value => {
console.log(value + " bar", bar);
});
bar = 1;
what if ?
const fs = require("fs");
let bar;
function someAsyncApiCall(callback) {
setTimeout(()=>{callback('setTimeout')}, 0)
setImmediate(callback, "immediate");
}
someAsyncApiCall(value => {
console.log(value + " bar", bar);
});
bar = 1;
what if?
const fs = require("fs");
let bar;
function someAsyncApiCall(callback) {
setTimeout(()=>{callback('setTimeout')}, 0)
setImmediate(callback, "immediate");
process.nextTick(callback, "nextTick");
}
someAsyncApiCall(value => {
console.log(value + " bar", bar);
});
bar = 1;
what if ?
const fs = require("fs");
let bar;
function someAsyncApiCall(callback) {
fs.readFile('', ()=>{
setImmediate(callback, "immediate");
setTimeout(()=>{callback('setTimeout')}, 0)
process.nextTick(callback, "nextTick");
})
}
someAsyncApiCall(value => {
console.log(value + " bar", bar);
});
bar = 1;
wait for your explaination
haha
附送一张nodejs event loop 生命周期图:
image.png
then how about?
let a ;
setTimeout(()=>{
console.info(a)
},0)
setImmediate(()=>{
a=1
})
And ?
let a ;
setTimeout(()=>{
console.info(a)
},0)
process.nextTick(()=>{
a=1
})
