I love TypeScript.  I use it whenever I can.  That said, sometimes it can be…  interesting.  Today, out of the blue, I got the typescript error in code that used to work:

[06:53:30]  typescript: src/mycode.ts, line: 57 
            Property 'video' does not exist on type 'number | (<U>(callbackfn: (value: Page, index: number, 
            array: Page[]) => U, thisA...'. Property 'video' does not exist on type 'number'. 

The code looks like:

return _.chain(pages)
        .filter((s, sIdx) => s.video || s.videoEmbedded)
        .map((s, sIdx) => {
            if (s.video) { ... }

Can you spot the ‘error’?

The problem is that s.video || s.videoEmbedded isn’t returning a boolean. It’s return a truthy value, but not a boolean. And the lodash typescript developers made a change 1 month ago that meant that filter() would only accept booleans, not any truthy value. And the lodash typescript developers are finding that fixing this becomes very complicated and complex. See the full conversation here:

https://github.com/DefinitelyTyped/DefinitelyTyped/issues/21485

(Open issue at time of writing. Please leave me feedback or message me if you see this bug get resolved)

The workaround/fix is to just make sure it’s a boolean. E.g. use !! or Boolean(..) or:

return _.chain(pages)
        .filter((s, sIdx) => s.video !== null || s.videoEmbedded !== null )
        .map((s, sIdx) => {
            if (s.video) { ... }