undefined items from array in TypeScript for loop

Question

When I'm setting an anonymous callback function on some REST service, I'm getting a very strange behavior where if I simply console.log the result of the REST service, I get the expected payload (an array of Objects). However, when I run a loop over the same array and try get some key's value, I get a error because apparently the item is undefined

...
callback: (result) => {
    console.log(result); // outputs [{text: 'foo'}, {text: 'bar'}]
    for(let item of result){
        console.log(item.text); // error can't read text of undefined
        console.log(item); // HOWEVER... this works... :/
    }
}

Any ideas? There must be some kind of async behavior happening, but I can't figure it out.

Thanks!

Everything seems to work as expected. Are you sure the response is really as in the comment? — martin
– martin, Commented Jul 7, 2016 at 8:08
use item['text'] instead of item.text. It should surely works... — Ajay
– Ajay, Commented Jul 7, 2016 at 8:15
Is this your actual code or you simplified it for the question? Have you debugged it using breakpoints? — Nitzan Tomer
– Nitzan Tomer, Commented Jul 7, 2016 at 8:18

basarat · Accepted Answer · 2016-07-07 08:47:54Z

2

You most likely have a malformed array. Here is an example that demonstrates the problem:

// Malformed array
const result = [
    {text: 'foo'},
    {text: 'bar'},
]
result.length = 3;

// Your code
console.log(result); // outputs [{text: 'foo'}, {text: 'bar'}]
for(let item of result){
    console.log(item.text); // error can't read text of undefined
    console.log(item); // HOWEVER... this works... :/
}

Fix

Filter out the empty items.

More tips

console.log will print an empty line for unreasonable stuff. You might not be seeing that in your debugging
Read up on JavaScript array holes
Enjoy life 🌹

answered Jul 7, 2016 at 8:47

basarat

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1 Comment

JobaDiniz Over a year ago

this is weird... and does not make much sense

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