How to Avoid 'Cannot Read Property of Undefined' Errors

How to avoid 'cannot read property of undefined' errors?

Update:

• If you use JavaScript according to ECMAScript 2020 or later, see optional chaining.
• TypeScript has added support for optional chaining in version 3.7.

// use it like this
obj?.a?.lot?.of?.properties

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Solution for JavaScript before ECMASCript 2020 or TypeScript older than version 3.7:

A quick workaround is using a try/catch helper function with ES6 arrow function:

function getSafe(fn, defaultVal) {
  try {
    return fn();
  } catch (e) {
    return defaultVal;
  }
}

// use it like this
console.log(getSafe(() => obj.a.lot.of.properties));

// or add an optional default value
console.log(getSafe(() => obj.a.lot.of.properties, 'nothing'));

Unable to fix cannot read properties of undefined

Based on a comment above, indicating that the error is thrown here:

return results.filter((quote) => {

If results is undefined then you can't "filter" anything on it. What should filteredItems return in that case? For example, you can perform null checking to return null:

return results?.filter((quote) => {

Or perhaps you could conditionally return an empty array:

return results ? results.filter((quote) => {
  //...
}) : [];

React Context. how to avoid Cannot read properties of undefined error before having a value

Your clickedImg starts out as the empty string:

const [clickedImg, setClickedImg] = useState("");

And in the consumer, you do:

const [slideIndex, setSlideIndex] = useState(clickedImg.id);

So, it takes the value of clickedImg.id on the first render - which is undefined, because strings don't have such properties. As a result, both before and after fetching, slideIndex is undefined, so after fetching:

albums ? albums[slideIndex].url : "no data"

will evaluate to

albums[undefined].url

But albums[undefined] doesn't exist, of course.

You need to figure out what slide index you want to be in state when the fetching finishes - perhaps start it at 0?

const [slideIndex, setSlideIndex] = useState(0);

How to prevent Cannot read property example of undefined when trying to check if said property is undefined

Your problem isn't that clicks is undefined; it's that your data variable is undefined. Check out this code snippet:

var data;
var playerid = idd.toString();
fs.readFile("data.json", function(err, data){
  json = JSON.stringify(data);
  data = JSON.parse(json);
})
if(data[playerid].clicks === undefined){ // <-- BOOM

The var data declares the variable, and it starts off holding the value undefined. You are populating data with your JSON.parse, but that's happening asynchronously in a callback. Therefore the if statement code runs before that data assignment.

You'll want to make sure that you're reading data after it has been populated by putting your code in the callback or using Promises and awaiting the asynchronous file read.

Here's an example of how you can use an async function (keeping in mind that the caller of this function also has to await it):

async function clickeridoo(idd){
  const playerid = idd.toString();
  const fileContents = await fs.promises.readFile("data.json");
  const data = JSON.parse(fileContents);
  // TODO: handle case where `data[playerid]` is undefined
  data[playerid].clicks = (data[playerid].clicks || 0) + 1;
  await fs.promises.writeFile("data.json", JSON.stringify(data));
  return data[playerid].clicks.toString()
}

How can I avoid Cannot read property error when the element isn't exist?

Make your code look something like this:

var element = $(document).find(".chosen-single:not(.chosen-default)  > span:first-child")[0];

if(element) {
   category = element.outerHTML
}

it's seems that sometimes the element you are searching is missing, so the result is undefined, to avoid this issue, check if found what you queried for

cannot read property 0 of undefined error

To avoid the error, replace

for (var i = 0; i < 5; i++) {

by

for (var i = 0; i < lista_carpetas_ok2.length; i++) {

This might also help to avoid the code to stop at certain folder.

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Regarding the error
> Info Cannot read property 'getRange' of null"

This occurs because the spreadsheet hasn't a sheet named fechas
to avoid this error you might add

if(!SpreadsheetApp.open(file).getSheetByName("fechas")) break;

above of

var sms = SpreadsheetApp.open(file).getSheetByName("fechas").getRange("A2").getValue();

if(

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Debugging tips:

• To verify that the correct values are assigned to lista_carpetas_ok2, add console.log(JSON.stringify(lista_carpetas_ok2)); just below lista_carpetas_ok2 declaration so you can review the values assigned to the variable.

• To have more informative logs when an error occurs, instead of

    } catch (e) {
      Logger.log(e.toString()); //  or console.log(e.message);
    }
  

  use

  } catch (e) {
      console.log(e.message, e.stack);
  }
  

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