[00:00] Let's imagine that we have 2 functions that do very, very similar things. We have join names here, which takes in a value which is unknown. This could be a value coming from anywhere, coming from search params, coming from user inputs. And it checks to see if it's an array, first of all, then it checks to see if every single item is a string

[00:19] inside them. And then if this works, then it joins them all together. We also have something that has pretty much the same logic in createSections, which takes in a value of unknown, does the same check, array.isArray, checks every single item, and then maps over it, and basically maps it with a section. So reformats the text. We've got some

[00:38] tests making sure they behave as expected and there's no errors in this file. But there's a bit of duplication happening here. We can see it just here. This duplication is a little bit awkward because we've got 2 pieces of logic that look exactly the same. Why not create a reusable type

[00:55] guard to basically grab that logic and just put it in a function somewhere so we can reuse it hundreds of times. That's your job, is to make sure everything keeps passing here, but we refactor it out to its own function to see if we can just reduce some of this duplication. Good luck!

Consider the following code:


```typescript
const joinNames = (value: unknown) => {
  if (Array.isArray(value) && value.every((item) => typeof item === "string")) {
    return value.join(" ");
  }


  throw new Error("Parsing error!");
};


const createSections = (value: unknown) => {
  if (Array.isArray(value) && value.every((item) => typeof item === "string")) {
    return value.map((item) => `Section: ${item}`);
  }


  throw new Error("Parsing error!");
};
```


In this example, we have two functions, `joinNames` and `createSections`, that perform similar checks on an unknown input value. Both functions first verify if the value is an array using `Array.isArray`, and then check if every item in the array is a string using `array.every(item => typeof item === "string")`.


The duplication lies within the type guard logic. Both functions have the same checks to determine if the input value is an array of strings.


This duplication can be extracted into a reusable type guard function to improve code maintainability and reduce repetition.


## Challenge


Your challenge is to create a reusable type guard that will replace the duplicate logic in the `joinNames` and `createSections` functions. The type guard function should take an unknown value as input and return a boolean indicating whether the value is an array of strings.


Practice refactoring duplicate type guard logic into a reusable function to improve code maintainability and reduce repetition.

[00:00] The solution here is amazingly simple. This used to be a much more complicated problem to solve, but since TypeScript 5.5, which as of the time of recording has just been released, we can now do this. I can just copy this logic into here. I can say const isArrayOfStrings, and then I'll just

[00:19] return this value here. Obviously, we're getting some errors because value is not defined. So let's define it here saying value is a type unknown. And now I can just replace the logic inside here and just say isArrayOfStrings calling it on value.

[00:35] And it works. Now, this is amazing to me. I mean, to you watching this in the future, this might not be so amazing because TypeScript will have had this for a while. But what's happening here is if we hover over isArrayOfStrings here, then you can see that we have the function parameter just there.

[00:52] In fact, I'll zoom in a touch. And we have a return type here, which looks very interesting. We have valueUnknown as the parameter, and the return type is valueIsStringArray. This is what's called a type predicate. And type predicates are something which are quite advanced in TypeScript. We will look at them, I think, probably in maybe the

[01:12] last chapter of this. And what's going on here is that TypeScript is inferring the fact that we're returning this logic inside here. We're doing some narrowing inside there. And it's inferring the fact that this function, when it returns true, will mean that value is inferred as an array of

[01:30] strings. So when we call it on here like this, it means that value is an array of strings. If we were to do the reverse, if we say it's not an array of strings, then value inside here would still be unknown. But outside of those brackets, value would be an array of strings. So it's like we're

[01:48] able to recapture some of this narrowing logic, put it inside a function, and it just works. You are, of course, allowed to add your own to here. And actually, 1 thing you'll notice is if you add a return type here, if you add Boolean as the return type, then everything actually stops working. If we hover over this, we can see value unknown

[02:08] returns a Boolean. And because it's not returning any special awareness of what value is supposed to be here, it means that value is still unknown here. So this is a good argument for actually not annotating your return types because if you're adding this interesting narrowing logic here, then your function is going to be automatically

[02:28] inferred as a type guard. Really, really clever stuff.

TypeScript 5.5 has introduced a remarkably simple solution to this problem that used to be much more complicated.


Create an `isArrayOfStrings` function that takes the original logic from the `joinNames` and `createSections` functions that returns a boolean indicating whether the value is an array of strings:


```typescript
const isArrayOfStrings = (value: unknown) => {
  return (
    Array.isArray(value) && value.every((item) => typeof item === "string")
  );
};
```


Then inside of the `joinNames` and `createSections` functions, replace the original type guard logic with a call to the `isArrayOfStrings` function that passes in the `value` parameter:


```typescript
const joinNames = (value: unknown) => {
  if (isArrayOfStrings(value)) {
    return value.join(" ");
  }


  throw new Error("Parsing error!");
};


const createSections = (value: unknown) => {
  if (isArrayOfStrings(value)) {
    return value.map((item) => `Section: ${item}`);
  }


  throw new Error("Parsing error!");
};
```


Now TypeScript is able to infer the type of `value` based on the return value of the `isArrayOfStrings` function. If the function returns `true`, TypeScript narrows the type of `value` to `string[]` within the `if` block.


## Understanding Type Predicates


Let's take a closer look at what's happening under the hood.


If we hover over the `isArrayOfStrings` function, we'll see something interesting:


```typescript
// hovering over isArrayOfStrings shows:
const isArrayOfStrings: (value: unknown) => value is string[]
```


The return type of the function is a type predicate: `value is string[]`. Type predicates are an advanced TypeScript feature that allows us to capture type narrowing logic within a function.


When `isArrayOfStrings` returns `true`, TypeScript infers that `value` is an array of strings. This means that within the `if` block, `value` is treated as `string[]`. If the function returns `false`, `value` remains of type `unknown`.


## The Importance of Inferred Return Types


Note that if we explicitly set the return type to `boolean`, TypeScript no longer infers the type predicate:


```typescript
const isArrayOfStrings = (value: unknown): boolean => {
  // ...
};


// hovering over isArrayOfStrings shows:
const isArrayOfStrings: (value: unknown) => boolean
```


This is a good argument for letting TypeScript infer the return type of functions that contain type narrowing logic.


Simplified Type Guards with TypeScript 5.5

Reusable Type Guards

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Reusable Type Guards

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