Could you provide an example value of the account html as well as any css associated with the id theTable? Just using the code above I'm unable to recreate this issue.

When you say that the array is undefined, do you mean that stringResult is undefined in the array function or is it that item[arrayFields] is undefined? Also, would you be able to share the entire LWC component? If so, I'd be happy to try running it in my own SF environment to make sure I am not missing something.

If you are only using JSON.stringify to view the contents of the variable, then I would recommend moving JSON.stringify from line 5 to the console log statement: this.stringResult = this.sectionInvLineItems; console.log( 'stringResult 2 ' + JSON.stringify(this.stringResult)); //returns array data This will ensure that the variable is an array instead of a string when you call the .filter method.