JavaScript Arrow函数语法

Question

I am studying JavaScript and have been learning how to use arrow function syntax and have come across this problem that I don't understand thoroughly and I feel this would be the best place to ask my question since I haven't found the exact answer I'm looking for on this issue, yet. So, here is the code.

let bigNumbers = [100, 200, 300, 400, 500];

let smallNumbers = bigNumbers.map(num => num / 100);

Now my question is, where does num come from? Is this a bucket to store the new values being mapped when running the code? I understand that we create a new variable that will return a new array since elements from the array have been modified? I'm not quite sure, which is why I need clarification. Thank you for your time.

Answer 1

num is simply an argument of your arrow function:

bigNumbers.map(num => num / 100)

...is shorthand for, and functionally equivalent to:

bigNumbers.map((num) => {
    return num / 100;
});

In your example, using the Array map() method, the num argument is the item in the array being iterated over in that instance.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

Answer 2

num is a function argument (or a Method property in your case; .map() being a Method of Array.prototype ).

num ... You can name it whatever you want.

What's basic here is what it represents . If you take a look at some documentation like https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

 var new_array = arr.map(function callback(currentValue[, index[, array]]) { // Return element for new_array }[, thisArg]) 

you can see that the first argument represents the currentValue , in other words the current iterating value.

let smallNumbers = bigNumbers.map(num => num / 100);

which translates to

let smallNumbers = bigNumbers.map((num) => {
    return num / 100;
});

which further translates to

var smallNumbers = bigNumbers.map(function(currentValue) {
    return currentValue / 100; // currentValue is the currently iterating value
});