在JavaScript中，如何将值从未命名函数传递给另一个未命名函数？

Question

I have two functions, one of which checks if an image loads:

var img = document.createElement('img');
img.src = "xxx";
img.onload = function() {
    var dimensionValue = 'loaded';
};
img.onerror = function() {
    var dimensionValue = 'failed';
}

A subsequent function executes when a user clicks a button, like so:

var trackClick = function(url) {
  console.log(dimensionValue);
};

I am trying to log the value of dimensionValue at the point the trackClick function is called (from an a link). However, despite the first function succesfully printing out the dimensionValue, when the second function is triggered it contains no value or it fails to indicate that dimensionValue does not exist.

I have also attempted to place the trackClick within the onload / onerror functions, but then a message displays informing that there is no function called trackClick

So, my question is how to pass a value from one unnamed function to another anonymous function in JS?

Answer 1

Your variable might not be able to be accessed by the other function because it is defined in another function.

Try:

var dimensionValue;
var img = document.createElement('img');
img.src = "xxx";
img.onload = function() {
    dimensionValue = 'loaded';
};
img.onerror = function() {
    dimensionValue = 'failed';
}

I think you could also do:

var trackClick = function(url) {
    var img = document.createElement('img');
    img.src = "xxx";
    img.onload = function() {
        console.log('loaded');
    };
    img.onerror = function() {
        console.log('failed');
    }
}

If this doesn't work, please post all of your code for further assistance.

Answer 2

By declaring dimensionValue outside of your functions, you widen the scope that can access it. Assuming your trackClick function is in the same scope, it should have access to dimensionValue .

var dimensionValue
var img = document.createElement('img');
img.src = "xxx";
img.onload = function() {
    dimensionValue = 'loaded';
};
img.onerror = function() {
    dimensionValue = 'failed';
}