Why use -1 when looping backwards in javascript?

Question

Here's the code:

var john = ['John', 'Smith', 1990, 'teacher', false, 'blue'];

for (var i = john.length - 1; i >= 0; i-- ) {
    console.log(john[i]);
}

I am trying to understand why use -1 in the declaration instead of using:

for(var i = john.length; i>-1; i--){
    console.log(john[i]);
}

Which to me makes more sense because the index i would have the value of the array which is 6 but since arrays starts with 0, it will not execute index 0 and therefore, for it to be executed, the condition has to be greater than -1.

Sorry I'm sort of new to programming.

The highest index of an array of length 3 is 2: 0,1,2. — Felix Kling
– Felix Kling, Commented Jan 17, 2020 at 22:15
Try running both versions of the code yourself and see what happens. — John Montgomery
– John Montgomery, Commented Jan 17, 2020 at 22:16
@FelixKling I am aware of that but why use john.length -1? I did run the code and there was an undefined value before it executes the array. Why is that so? — Ai Hime
– Ai Hime, Commented Jan 17, 2020 at 22:51
Because if i = john.length, then i = 6 (in your example) and john[6] is undefined because the highest index is 5 (john[5] returns 'blue'). Simple reproduction: console.log([1,2,3][2]); console.log([1,2,3][3]);. — Felix Kling
– Felix Kling, Commented Jan 17, 2020 at 23:23

Pointy · Accepted Answer · 2020-01-17 22:14:45Z

1

In the first iteration, i would be john.length, so the reference to john[i] would be past the end of the array. Array indexes go from 0 to length - 1.

Of course i > -1 is just as good as i >= 0 if you prefer it.

answered Jan 17, 2020 at 22:14

Pointy

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Comments

Nina Scholz · Accepted Answer · 2020-01-18 09:50:25Z

1

For looping backwards, you might use a different pattern, which uses the truthy/falsy characteristic of a number in a condition.

This approach uses a check for truthiness and supports zero based indices of arrays.

var john = ['John', 'Smith', 1990, 'teacher', false, 'blue'],
    i = john.length;

while (i--) {
    console.log(john[i]);
}

edited Jan 18, 2020 at 9:50

answered Jan 17, 2020 at 22:22

Nina Scholz

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