JS callback function uses parameters

This time I will bring you the parameters used by the JS callback function. What are the precautions for using the parameters of the JS callback function? The following is a practical case, let's take a look.

Before talking about the callback function, you might as well look at a piece of code first. I believe that students with some basic knowledge of js can understand its meaning:

document.getElementById('demo').click=function(){
  alert(1);
};

This code is actually an event callback. Write it like this In fact, it is relatively vague. Let’s take a look at the following code.

document.getElementById('demo').addEventListener('click',function(){
    alert(1)
});

The two pieces of code actually do the same thing. The only difference is the way of writing. Let’s look at thisaddEventListener('eventName ',callback),addEventListenerThis function has two parameters, the first is the event name, and the second parameter is actually the callback function. According to the callback function in the book, the function Since the parameter in can be a variable, it can also be a function. Maybe everyone is still confused about returning functions at this point. Let's look at the next example.

function demo(a,b,callback){
    let c=a+b;
    callback(c);
};
demo(1,2,function(c){
    alert(c);//3
})

This code defines a demo function. This function has three parameters a, b, callback. We declare a local variable c inside this function, and then execute our callback (callback function), then Come down and execute the demo function

The three parameters of this function are as above, and the value 3 pops up in the callback function. This is a simple callback function. To truly understand the meaning of the callback function, I actually think it depends on understanding its purpose. Only by understanding its purpose can we truly understand it.

The most common application scenario for callback functions is asynchronous operations. Because for asynchronous operations, we don’t know when the operation will end, then the code we execute subsequently cannot follow the proper process.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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