JSON array iteration skips first element - NodeJS - arrays

Question

Ive got this JSON markup:
"Categories": [
{"name": "a", "id": "1"},
{"name": "b", "id": "2"},
{"name": "c", "id": "3"},
{"name":"d", "id": "4"},
{"name":"e", "id": "5"},
{"name": "f", "id": "6"},
{"name": "g", "id": "7"},
{"name": "h", "id": "8"}
]
Ive got a setInterval going over each of these categories and making a new Promise for each one.
For some unknown reason, it always skips the first element and exists with an out of bounds exception
var i = 0;
var id = setInterval(function(){
if (i == categories.length){
clearInterval(id);
}
client.itemSearch({
category: categories[i].id,
catName: categories.name,
}).then(function(results){
console.log("From category - " + categories[i].name + "\n" +
"Title: " + results[0].Title);
},function(err) {
console.log("error at " + categories[i].name);
});
i+=1;
}, 1000);

Answer 1

The problem is that you're referencing i in your promise callbacks and i changes every second. So your callbacks are being executed sometime in the future when i === categories.length, which would be an invalid index.
Since you already have a closure, just store the current category object in a variable outside of your promise callbacks and reference that instead.
You're also letting the function continue when i === categories.length which will cause an invalid index access.
Here's an example of a solution that fixes the aforementioned issues:
var i = 0;
var id = setInterval(function() {
if (i === categories.length) {
clearInterval(id);
return;
}
var category = categories[i];
client.itemSearch({
category: category.id,
catName: category.name,
}).then(function(results) {
console.log("From category - " + category.name + "\n" +
"Title: " + results[0].Title);
}, function(err) {
console.log("error at " + category.name);
});
i += 1;
}, 1000);
Lastly, you originally had catName: categories.name but I changed it to catName: category.name as I assumed that may have been your original intention.