var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
// and store them in funcs
funcs[i] = function() {
// each should log its value.
console.log("My value:", i);
};
}
for (var j = 0; j < 3; j++) {
// and now let's run each one to see
funcs[j]();
}
它会输出以下内容:
我的值:3
我的值:3
我的值:3
但我想要输出:
我的值:0
我的值:1
我的值:2
当函数的延迟由事件侦听器引起时,就会出现同样的问题:
var buttons = document.getElementsByTagName("button");
// let's create 3 functions
for (var i = 0; i < buttons.length; i++) {
// as event listeners
buttons[i].addEventListener("click", function() {
// each should log its value.
console.log("My value:", i);
});
}
<button>0</button>
<br />
<button>1</button>
<br />
<button>2</button>
...或异步代码,例如使用Promises:
// Some async wait function
const wait = (ms) => new Promise((resolve, reject) => setTimeout(resolve, ms));
for (var i = 0; i < 3; i++) {
// Log `i` as soon as each promise resolves.
wait(i * 100).then(() => console.log(i));
}
这也可以在 for in
和 for of
循环中看出:
const arr = [1,2,3];
const fns = [];
for (var i in arr){
fns.push(() => console.log("index:", i));
}
for (var v of arr){
fns.push(() => console.log("value:", v));
}
for (const n of arr) {
var obj = { number: n }; // or new MyLibObject({ ... })
fns.push(() => console.log("n:", n, "|", "obj:", JSON.stringify(obj)));
}
for(var f of fns){
f();
}
这个基本问题有什么解决方案?
let
代替var
通过在每次循环运行时创建一个新的作用域,为每个函数创建单独的作用域来解决此问题。其他各种技术使用额外的函数完成相同的操作。 - Costa Michailidis