Error when comparing an object with itself

Question

I'm trying compare an value with the next value in the array, but when the loop is finished, the code that is after is not executed.

var json = [{'id':1},{'id':2},{'id':3},{'id':4},{'id':4},{'id':5},{'id':5}];

for(var i = 0; i < json.length; i++){
    if(json[i].id == json[i+1].id){
        console.log("Equal");
    }
}

console.log('Something'); //This code is not executed

I realized that the problem is this part [i+1]. When I remove the number 1, the code is executed normally.

Does anyone know why this happens?

In your head, try stepping through the loop one iteration at a time. What happens when `i` is 1 less than the array's length? `json[i+1]` will be undefined. If the array length is 3, then `i+1` will be `3`. `json[3]` is `undefined`, so you can't get the `id` from `json[3]`. — Cerbrus, Jul 09 '18 at 12:15
A look into the browser console would have given you a lead already. — CBroe, Jul 09 '18 at 12:16
@OlivierKrull: _"This question does not show any research effort; it is unclear or not useful"_ < That's what the downvote button says on its tooltip. — Cerbrus, Jul 09 '18 at 12:17
Duplicate of: https://stackoverflow.com/questions/31973278/iterate-an-array-as-a-pair-current-next-in-javascript — Cerbrus, Jul 09 '18 at 12:32

score 1 · Accepted Answer · answered Jul 09 '18 at 12:24

You need to have your loop condition as i < json.length-1 instead of i < json.length as having i < json.length as a condition will raise error of:

Uncaught TypeError: Cannot read property 'id' of undefined

And terminate the process so you do not get the last console.log()

var json = [{'id':1},{'id':2},{'id':3},{'id':4},{'id':4},{'id':5},{'id':5}];

for(var i = 0; i < json.length-1; i++){
    if(json[i].id == json[i+1].id){
        console.log("Equal");
    }
}

console.log('Something');

Sara. · Answer 2 · 2018-07-09T13:38:36.677

-2

You can add this condition in your if statement:

var json = [{'id':1},{'id':2},{'id':3},{'id':4},{'id':4},{'id':5},{'id':5}];

for (var i = 0; i < json.length ; i++) {
    if ((i+1) < json.length  && json[i].id == json[i+1].id) {
        console.log("Equal");
    }
}

console.log('Something');

edited Jul 09 '18 at 13:38

answered Jul 09 '18 at 12:58

Sara.

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Error when comparing an object with itself

2 Answers2