Why `--array.length` delete the last element of the array?

Question

In javascript, chrome console:

var test = [0, 1, 2, 3, 4]
> undefined
var lastIdx = --test.length
> undefined
lastIdx
> 4
test
> [0, 1, 2, 3]
var lastIdx = --(test.length)
> undefined
lastIdx
> 3
test
> [0, 1, 2]

As you already saw, I just want to get the last index of the array through the --array.length, but unfortunately the last element of the array get removed unexpectedly, really cannot understand how come this can/should happen, can someone explain?

-- does not mean just "one less than this number". It also decreases whatever you apply it to. — user2357112
– user2357112, Commented Nov 17, 2016 at 3:39

Jesse · Accepted Answer · 2016-11-17 03:46:50Z

3

--x decrements x and returns the new value. (x-- decrements x and returns the old value.) So --test.length reduces the length by 1.

If you just want to get last index without modifying the length, that would be test.length - 1.

answered Nov 17, 2016 at 3:46

Jesse

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1 Comment

Ruan Mendes Over a year ago

In other words, it's like saying array.length = array.length - 1

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