Javascript，这个函数中的 (n) 和方括号是做什么的？

Question

Just got a couple of questions to help me understand this script, as where it is OP, the explanation is pretty thin.

1) (n) is passed into the functions plusDivs and showDivs. How does this (n) work? Is it just an empty container to store the data in the function.

2) With this line x[i].style.display="none"; and x[slideIndex-1].style.display="block"; with the square brackets used, is the code inside the square brackets executed first? What are the square brackets doing or called in this instance.

Although functions are explained in online tutorials in a basic way, some of this information I'm asking is quite hard to find out so would appreciate some help .

Thanks Emma.

JS:

var slideIndex = 1;
showDivs(slideIndex);

function plusDivs(n) {
    showDivs(slideIndex += n);
}

function showDivs(n) {
    var i;
    var x = document.getElementsByClassName("mySlides");
    if (n > x.length) { slideIndex = 1 } 
    if (n < 1) { slideIndex = x.length };
    for (i = 0; i < x.length; i++) {
        x[i].style.display = "none"; 
    }
    x[slideIndex-1].style.display = "block"; 
}

HTML:

<button style="border:none; font-size: 22px;background-color:black; color:white; position:fixed; top:30%; left:5%" onclick="plusDivs(-1)">&#10094;</button>
<button id="rightbutton" style="border:none; font-size: 22px;background-color:black; color:white;position:fixed; top:30%; right:5%" onclick="plusDivs(1)">&#10095;</button>

Answer 1

In JavaScript, functions are portions of code that take caller-supplied information called "arguments" and use them in the code. The syntax of a function is

function (x, y) {
    return x + y;
}

If the above function were to be called:

(function (x, y) {
    return x + y;
})(1, 1)

The result would be 2 .

The [ brackets ] are subscript operators. In the context of arrays, they take a numeric index and return the element at that location:

[1, 2, 3][0] === 1

In the context of objects, they take a key and return the mapped value:

(({
     'foo' : 'bar'
})['foo']) === 'bar'

In your case, the function getElementsByClassName returns an array of elements, so the brackets denote array subscription and are used to retrieve an element of the array.

Answer 2

1) (n) is passed into the functions plusDivs and showDivs. How does this (n) work? Is it just an empty container to store the data in the function.

an integer passed into plusDivs() function, its mean by pressing buttons you pass "-1 and +1" integer as argument to your function and in your function it will add/contact it from you current slide index that will cause your slider moves to forward(next slide) or backward(prev slide)

2) With this line x[i].style.display="none"; and x[slideIndex-1].style.display="block"; with the square brackets used, is the code inside the square brackets executed first? What are the square brackets doing or called in this instance.

As i mentioned in comments they are for finding nth element inside an array. at first of your code you find all slides and cached them into a variable, so when you need to find nth slide youmost use something like this slide[slide number 2] (this is just pseudo code)

Answer 3

I don't think the answers in this case are very clear. The (n) and i in your question are just free variables. They could be any letter (eg i could be 'b' or some other letter).

They are just naming and referencing the var properties.