Cannot set property 'comment' of undefined inside .then()

Question

Why is obj[i] undefined inside .then()?

obj = [{'id': 1, 'name': 'john', 'age': '22', 'group': 'grA'}, {'id': 2, 'name': 'mike', 'age': '24', 'group': 'grA'}, {'id': 3, 'name': 'anne', 'age': '25', 'group': 'grB'}]

for (var i = 0; i < obj.length; i++) {
    console.log(obj[i]) // has the right value
    this.commentService.getAllComments(obj[i].id).then((res) => {
        ...
        console.log(obj[i]) // undefined ???
    })
}

Is there any posibility I can solve this situation and is it there an explanation why is undefined? Thank you for your time!

EDIT: The problem was that I was using var instead of let. Thank you!

Please provide a [mcve] of your issue or at least your full code. — , Apr 01 '19 at 14:06
normally this issue is when you use var and not let in the for loop. https://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example — epascarello, Apr 01 '19 at 14:08
Possible duplicate of [JavaScript closure inside loops – simple practical example](https://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) — nicholaswmin, Apr 01 '19 at 14:10
Possible duplicate of [JavaScript for loop index strangeness](https://stackoverflow.com/questions/2803351/javascript-for-loop-index-strangeness) — lleon, Apr 01 '19 at 14:19

score 0 · Answer 1 · edited Apr 01 '19 at 15:07

0

Using var will define variable on the wrapping function level. Since your code inside promise then callback is running after the for loop finished execution, you'll always get the i === obj.length. To avoid this, you can define block scope local variable, using let keyword.

for (let i = 0; i < obj.length; i++) { // let instead of var
    console.log(obj[i]) // has the right value
    this.commentService.getAllComments(obj[i].id).then((res) => {
        ...
        console.log(obj[i]) // has the right value
    })
}

edited Apr 01 '19 at 15:07

Vadim

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answered Apr 01 '19 at 14:22

Tenzolinho

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1

Why would that make a difference? – Nino Filiu Apr 01 '19 at 15:06

Cannot set property 'comment' of undefined inside .then()

1 Answers1