Logic in "-1" needed. for loop with array in javascript

Question

var vacationSpots = ['Paris', 'New York', 'Barcelona'];
for(var i = vacationSpots.length - 1; i >= 0; i--) {
console.log('I would love to visit ' + vacationSpots[i]);
}

Hello guys,my question is whats the logic in "-1" in the for loop to be reverse.i get it that for(var i = vacationSpots.length; i >= 0; i--) { helps you running Backwards. but what's the use of -1 in printing the items in array backwards?

• Charlie: If you would start at i=3, then it'll try to get the 4th item of the array, as arrays are 0-indexed. Since the length is 1-indexed (starts at 1), you have to do - 1 to make sure it is in range of the array.

Answer 1

Very simple ... array length is a count but indexing is zero based.

So if myArray length is 5 , the last index is 4 and myArray[5] doesn't exist.

So when iterating arrays by index you can't overshoot the last index which is length-1

• alexanderbird: Just to add to that, this applies whether you're looping forwards or backwards.
• Deniel Salvacion: i get it now. sorry im a newbie in javascript.well thank you so much for your help :)

Answer 2

(vacationSpots.length) is the length of the array but array index start from 0. So here the i value is set to 2 that mean the loop will execute 3 times

Initially i will set to 2, so vacationSpots[2] will be I would love to visit Barcelona', thenidecremented to 1 and output will beI would love to visit New York' & finally 0 and output will be I would love to visit Paris

• Deniel Salvacion: Thank you very much bro! i get it now :)

Answer 3

In addition

There is another way for reverse loop (without '-1'):

var vacationSpots = ['Paris', 'New York', 'Barcelona'];
for (var i = vacationSpots.length; i--;) {
 console.log('I would love to visit ' + vacationSpots[i]);
}

• vol7ron: You're right, I didn't confuse it, I just had a brain fart. I'll delete the comment. Thanks for putting up with my inebriation ;)

Answer 4

Index Array in Javascript start at 0, so you have vacationSpots[0]=Paris, vacationSpots[1]=New York, vacationSpots[2]=Barcelona. vacationSpots.length is 3 so you need to print 0 1 2. In general index goes from 0 to n-1, where n = number of items (length).