这个 for 循环数组是如何工作的？

Question

I just learn about array today but I don't actually understand this code I'm learning.

let units = [1, 10, 100, 1000, 20];

for(let i = 0; i < units.length; i += 1){
    console.log(units[i]);
}

How would console.log(units[i]); return 1 , 10 , 100 , 1000 , 20 and not 5 ?

Could you guys explain to me the way I will actually understand this?

Answer 1

units is an array.

units[0] gets you the first item in the array (1). units[1] gets you the next item in the array (10). and so on.

So as i is incremented with each loop iteration, you get the corresponding item from the list.

Answer 2

Just for your information.

This below is the initialization of list of numbers in a variable units

let units = [1, 10, 100, 1000, 20];

Now the loop begin.

for() is the loop.

Where 1st params let i=0 tell the starting point of the array.

2nd params i<units.length defined the length of array where the loop needs to be end

3rs params tell the index need to be increment each time the loop executed. For eg: when index i is 0, then value units[i] = 1 . After its execution the value of index i is incremented by 1 due to i++ as third param of for() loop.

The loop will continue until it meets the value of last index defined in second params that is i<units.length

for(let i = 0; i < units.length; i += 1){
    console.log(units[i]);
}

I hope it made you clear.

Answer 3

You are both declaring that a variable units exists and you are putting elements in the array with your statement

let units = [1, 10, 100, 1000, 20];

This has created an array with 5 elements; array elements are numbered (indexed) starting at zero :

| 1 | 10 | 100 | 1000 | 20 |     // array with elements in it
  0   1    2     3      4        // positions of elements

— the first element in the array is at position 0
that is, the item at units[0] is 1 , the item at units[1] is 10 , etc.

Now you start your loop, using the variable i as a number to use as an index into the array.

When i === 0 then units[i] is asking for the element at position units[0] , which is 1,
when i === 1 then units[i] is asking for the element at position units[1] , which is 10,
and so on.

So when you say console.log(units[i]); you are not printing the value of i ; you are getting the element in the array units that is at position i in the array and printing that .
console.log(units[i]) when i === 3 gets the item in the array at position 3, which is 1000 , so it prints 1000.