CODEWITHSUNDEEP
javascriptarrays
Ai Hime
Ai Hime

Reputation: 11

Why use -1 when looping backwards in javascript?

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.

Upvotes: 0

Views: 66

Answers (2)

Nina Scholz
Nina Scholz

Reputation: 386550

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]);
}

Upvotes: 1

Pointy
Pointy

Reputation: 413702

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.

Upvotes: 1

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