JavaScript array iteration - MDN example

Question

I was reading the re-introduction to JavaScript on the MDN website and came across this example in the Array section:

for (var i = 0, item; item = a[i++];){
   // Do something with item
}

Where "a[]" is an array being looped over.

I am confused about the value that "item" will have in its first iteration. As i=0 and item is at first undefined, then when it is assigned the value of a[i++] wouldn't the iteration start from i=1, which would mean that the iteration would start from the second element in the a[] array -> a[1], skipping over the first element a[0] entirely?

Note that as soon as the current element in the array is 0 or false, the loop will end. — Vlad
– Vlad, Commented May 13, 2013 at 16:24
This is one of the worst possible for loops I have seen in JavaScript before. You manage to save 1-2 characters. But sacrifice the usefulness of the index variable and readability. — vinczemarton
– vinczemarton, Commented May 13, 2013 at 16:58

Matt Ball · Accepted Answer · 2013-05-13 16:18:14Z

4

i++ is the post increment operator, which means that it increments i by 1 but evaluates to the old (non-incremented) value.

> i = 0
  0
> i++
  0
> i
  1

answered May 13, 2013 at 16:18

Matt Ball

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2 Comments

Halcyon Over a year ago

Note that in some languages ++i also exists, it does pre-incrementing, so it has the new value right away.

demee Over a year ago

++i also exists in JS if one would like to use it

Matt Ball · Accepted Answer · 2013-05-13 16:29:39Z

2

i++ is post increment (see other answers) and item will not be undefined, because the predicate (the second part in the for loop) is executed before each iteration.

for (var i = 0, item ; item = a[i++];) {

will evaluate to:

var i = 0;
var item;

item = a[i];  // loop
i += 1
if (!item) // exit loop
// loop body
// start again at loop

The problem with this syntax is, that if a value in a is falsy, the loop will terminate prematurely.

var a = [1,2,0,3,4];
for (var i = 0, item ; item = a[i++];) {
   console.log(item);
}

Will output "1 2" because "0" is falsy and the loop terminates

edited May 13, 2013 at 16:29

Matt Ball

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answered May 13, 2013 at 16:25

MofX

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1 Comment

vinczemarton Over a year ago

There are so many unexpectedly falsey values in JavaScript. Just look at this, for example.

user1796666 · Accepted Answer · 2013-05-13 16:18:31Z

1

i++ means that javascript reads the i value and then increments it

answered May 13, 2013 at 16:18

user1796666

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