我无法理解这段代码是如何工作的

Question

I am facing trouble understanding how exactly does the code return a value to randFace();

I have understood the use of rand(0,5), so that it can randomly generate a number which happens to be the length of the ["crown", "anchor",..] array.

I cannot understand the working of the code. Let's say, rand(0,5) returns 3 as a number, then I do get the idea that randFace() would return "spade", but how?

function rand(m, n) {
return m + Math.floor((n - m + 1)*Math.random());
}

// randomly returns a string representing one of the six
// Crown and Anchor faces

function randFace() {
return ["crown", "anchor", "heart", "spade", "club", "diamond"]
[rand(0, 5)];
}

Answer 1

Here's your code, rewritten for easier understanding. Does the same thing. Only extracted two variables and gave them names.

function randFace() {
  var faces = ["crown", "anchor", "heart", "spade", "club", "diamond"];
  var faceIndex = rand(0, 5);
  return faces[faceIndex];
}

Answer 2

When you do array[index] , you will get element at position index in that array. If there is not element, undefined will be returned as default.

You code is a shorter version of something like this:

 function rand(m, n) { return m + Math.floor((n - m + 1) * Math.random()); } function randFace(m,n) { var array = ["crown", "anchor", "heart", "spade", "club", "diamond"]; var index = rand(m,n); return array[index] } console.log(randFace(0,5)) console.log(randFace(10,15))

So when you do functionName(args) , function will be called first and the output will be passed to parent scope. So when you do [...][rand(0,5)] , first rand(0,5) will execute.

The output will be used as index value to fetch element in array.

As correctly commented by @ sergio tulentsev , in following line

return ["crown", "anchor", "heart", "spade", "club", "diamond"][rand(0,5)]

order of execution would be

1. initialize array: ["crown", "anchor", "heart", "spade", "club", "diamond"]
2. Then rand(0,5) and output will be passed to second []
3. Second [] will be executed and element will be fetched
4. fetched value will be returned.