Iterating through an array in a TypeScript for loop to handle undefined elements

Whenever I set an anonymous callback function on a REST service, I encounter a peculiar issue. If I simply use console.log to display the result of the service call, I am able to see the expected payload (an array of objects). However, when I attempt to iterate over this array using a loop and access the value of a key, I receive an error indicating that 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... :/
    }
}

Do you have any insights on what might be causing this issue? It feels like there might be some asynchronous behavior at play, but I'm unable to pinpoint it.

Appreciate your help!

Answer №1

Your array appears to be malformed, which is causing the issue you are experiencing. Here's an example showcasing the problem:

// Malformed array
const data = [
    {name: 'Alice'},
    {name: 'Bob'},
]
data.length = 3;

// Your code
console.log(data); // outputs [{name: 'Alice'}, {name: 'Bob'}]
for(let item of data){
    console.log(item.name); // error: cannot read property 'name' of undefined
    console.log(item); // However, this still works somehow.
}

Resolution

  • Remove the empty items from the array.

Additional Suggestions

  • When using console.log, it may display a blank line for invalid input that you might not notice during debugging.
  • Read about JavaScript array holes to better understand this behavior.
  • Remember to savor the moments in life 🌺

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