Javascript 检查图像是否存在，变量值不变

此代码检查是否存在图像

在onload函数中，我想用一个布尔值更改变量“control”

var control = 'a.jpg';
var image_name = control;
var image = new Image();

image.onload = function() {
    control = true; // Why this change does not happen outside of this function?
};
image.onerror = function() {
    control = false; // Why this change does not happen outside of this function?
};
image.src = image_name;

console.log(control); // The value is not changed in a boolean

但在函数之外，变量不变。为什么?

---

谢谢

这是因为

console.log（control）image

的

onload

或

onerror

函数之前调用code>

控件

变量会更改，但在更改前调用

日志

。您可以将代码修改为类似以下内容，以实现所需：

var control = 'a.jpg';
var image_name = control;
var image = new Image();

image.onload = function() {
    control = true;
    after();
};
image.onerror = function() {        
    control = false;
    after();
};
image.src = image_name;

function after(){
    alert(control);
    console.log(control);
}

---

您也可以在此处进行检查。

函数image.onload仅在加载图像后执行，执行可能需要500-1200ms

console.log（控制）在加载图像之前执行，这就是为什么总是显示为false

尝试在几秒钟延迟后打印控制值

var control = 'a.jpg';
var image_name = control;
var image = new Image();

image.onload = function() {
    control = true; // Why this change does not happen outside of this function?
};
image.onerror = function() {
    control = false; // Why this change does not happen outside of this function?
};
image.src = image_name;
setTimeout(function(){ 
console.log(control); // its work
},2000);

像这样更改代码结构

var control = 'a.jpg';
var image_name = control;
var image = new Image();

image.onload = function() {
    control = true; 
    nextProcess();
};
image.onerror = function() {
    control = false; 
    nextProcess();
};
image.src = image_name;
function nextProcess(){
  console.log(control);
}