There's only a single option to create a JS function

When I type a JS function to call it and the function doesn't exist, I see only one option to create it and that option is to create a local function. This means a function inside my function. var functionName = function() { throw new Error("Not implemented"); };

I using TypeScript. I would like to see an option to create a non local function and it looks like this:

functionName = (): void => {
}