Write one generic function instead of five near-identical ones

I've now written the same lookup function three separate times, once for every data shape, because TypeScript won't let me share just one version of it.

Intermediate to advancedRead 7 minDoing: under an hour8 steps

▶ Start

What you'll have at the end

One generic function replacing three near-identical ones, with the same type safety on each call

Before you start

You need

Three separate functions, one per data shape, that each loop over an array looking for one item whose named field matches a value you pass in.

Not covered

Replacing the array scan itself with a Map or a database index for faster lookups, and any generic version that returns more than one matching item.

Leans on

8 steps

Where to go next

Deriving a smaller type from a big interface with Pick

For trimming a large interface down to just the fields a caller needs, the sibling generic pattern to the lookup function built here.

Narrow an unknown JSON value before you trust it

For validating a whole value's shape once it arrives from outside the type system, the same problem the runtime key check here solves for a single key.

Checked 22 Aug 2026

Part of the TypeScript cookbook