TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'

Question

I am trying to create a chrome extension in typescript.

I have the following code, where I try to send a message to contentscript from the background script.

// background script 
chrome.tabs.query({active: true, currentWindow: true}, function(tabs){
   chrome.tabs.sendMessage(tabs[0].id, {action: "open_dialog_box"}, function(response) {
      console.log(response);
   });  
});

but typescript is giving the following error

ERROR in /Users/shobi/Projects/Chrome Extensions/src/js/background.ts(8,37)
      TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.

I tried converting the id to an integer using parseInt(), but still its not preventing the error.

Answer 1

Chrome's tab .id is possibly undefined (because it's defined as optional in the type definitions), which means when using strictNullChecks you have to either use a non-null assertion:

tabs[0].id!

Or write code to ensure that id is not null:

if (tabs[0].id) { // send message

Answer 2

I'm guessing, but maybe typescript thinks that your tabs array could be empty (and thus not have an item at index [0]) - which is why it's type is inferred as number or undefined?

If this is the case, you should check that tabs[0] exists before trying to access the id property on it.