[00:00] In this exercise, we have a string, which is hello world, and then we're trying to say string.replaceAll hello goodbye, but TypeScript is yelling at us. Property replaceAll does not exist on type hello world. Your job is to try to work out why this is happening and what we can change in our tsconfig.json

[00:19] to try to make this work. There are two possible solutions that you can use in your tsconfig.json, but that is the area that you need to look at first of all, where it is just here. Good luck.


In this exercise, we have a simple operation that you might expect to work without issues.

We're working with a basic string, "Hello, World!", and we're attempting to replace all instances of "Hello" with "Goodbye" using the `replaceAll` method:

```tsx
const str = "Hello, world!";

str.replaceAll("Hello", "Goodbye"); // red squiggly line under "replaceAll"
```

Unfortunately, TypeScript isn't happy with us and raises an error:

```tsx
// hovering over "replaceAll" shows:
Property 'replaceAll' does not exist on type '"Hello, world."'. Do you need to change your target library? Try changing the `lib` compiler option to es2021 or later.
```

## Challenge

Your task is to figure out why the error is occurring and how to resolve it by modifying the `tsconfig.json` file.

There are two possible solutions that you can use in your `tsconfig.json`.

For reference, here is the current `tsconfig.json` file:

```json
{
  "compilerOptions": {
    "target": "ES5",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "esModuleInterop": true,
    "outDir": "dist",
    "rootDir": "src",
    "strict": true,
    "skipLibCheck": true,
    "isolatedModules": true,
    "moduleDetection": "force",
  }
}
```

Resolve an error caused by the lib compiler option.

[00:00] Okay, so the reason that this was breaking first of all is because we had our TS config setting on Target set to es5 now when we have targets set this indicates What kind of version of JavaScript we are targeting es5 we have es

[00:18] 2016 es2017 and we can choose es3 I think but I think this will error possibly in future versions of JavaScript or typescript. Sorry. So the one that I tend to recommend choosing is yes 2022 and now because we've got it sent to es 2022 if we look at string to replace all the error has actually gone away

[00:40] But how on earth does that work? So because if we had it set to es5 Let's say and then we go back to string to replace all property replace All does not exist on type. Hello world. Do you need to change your target library? Try changing the lib compiler option to es2021 or later

[00:57] Well, how does TypeScript like what is it doing like to understand that replace? All is a valid on one target, but not on another well replace all was added in a certain Time in JavaScript's history. It looks like yes

[01:13] 2021 is the one and so what TypeScript does is you can operate it at various versions of JavaScript and it will give you basically warnings if you're using the wrong methods So if we look at string here, if we actually like look up for the string definition

[01:29] I think this will be easier if we actually fix the TS config or actually let's say we say String dot replace because this has been in JavaScript for a long time Let's come on click into that and now we can see that we end up in lib dot es 2015 dot symbol dot well known dot D dot TS

[01:48] And let's have a look at the files in here. You can see that there are a bunch of dot D dot TS files which are Declaration files as we're used to seeing that describe a bunch of different things in TypeScript and you can see that there's lib ES 2017 dot full dot D dot TS in pull

[02:08] object shared memory string type to raise a bunch of different features that you can add to your TypeScript and This is what target is doing. It's saying okay. You want to target ES 5? Okay. Well, you just get the stuff That's in lib dot ES 5 dot D dot TS and you can see inside here

[02:28] We're saying declare function which puts something into the global scope and now we have access to parse int if we didn't Specify any of this stuff then we would basically get if we look at our TS config Just then You can I think actually specify like no lib true here

[02:46] Yeah, no lib true and what we get is we would get like nothing here So replace doesn't exist anymore And if we were to look at all the methods on string we literally just have at it seems I'm sure where that's come from actually, but like so lib is basically the place where TypeScript decides

[03:06] What global methods are available on strings functions Basically describes what the language is doing and because the language has evolved over time That means that when we specify different targets We can specify different features added into the global scope of TypeScript again. It doesn't change anything at runtime

[03:26] It's just changing what's available to you on the type level So we have targets here, but we also have a second option The second option is to use lib and lib if I look at the second solution here, which is TS config here Inside this one. We're specifying targets, but we're also

[03:44] Specifying lib here and lib lets us drill down a little bit more into things that we can add Including a DOM type here, which we'll look at soon so each inside here you can be really really specific with what you choose and If you don't specify lib then it sort of defaults to the target that you've got specified here

[04:04] So if you have lib, do you need target is a final thought I've sort of not quite worked this out for myself But every time I talk to the TypeScript team the TypeScript team say oh, yeah Always specify a target always specify targets and I'm not quite sure like I think that's how that's the assumption

[04:23] That TypeScript makes is that you're specifying a target and a lib and in general if you're working With node for instance, you want that target and lib to be yes 2022 which is usually the latest for node and if you're working on bundling TypeScript for a web application

[04:40] Usually the bundler itself that you're using will handle this stuff. But using targets and lib Yes, 2022 is pretty safe. And yeah, I think that's everything we need to cover there So just to summarize we have lib.d.ts Which are a bunch of different files which we command click into string here

[04:59] We can see tons and tons of stuff that basically describe what JavaScript was like at that point in time and you can target different versions of JavaScript by specifying lib and target in your TS config


The reason we were getting the error is because the `target` setting in the `tsconfig.json` `compilerOptions` was set to `ES5`, which doesn't support the `replaceAll` method.

## Updating the `target` Setting

Changing the `target` setting to `ES2022` resolves the error:

```tsx
{
  "compilerOptions": {
    "target": "ES2022",
    // rest of config as before
```

In VS Code, `command + click`ing on `replaceAll` in the editor will take you to the `lib.d.ts` file's type declarations.

There are a variety of `lib.d.ts` files that ship with TypeScript, each corresponding to a specific version of JavaScript.

## Specifying a `lib` Setting

In addition to updating the `target`, we can also add a `lib` option to `tsconfig.json`:

```tsx
{
  "compilerOptions": {
    "target": "ES2022",
    "lib": ["ES2022"],
    // rest of config as before
```

Specifying the `lib` option also lets us drill down into the specific features and libraries we want to include in our project, which we will see more of later.

For example, you could specify a `target` of `ES5` and but bring in `ES2022` features by specifying `lib` as `["ES2022"]`.

However, the general advice from the TypeScript team suggests that you specify both `target` and `lib` in your configuration to `ES2022` as a safe course of action.

Changing the target and lib compiler options in tsconfig.json affect the JavaScript features that TypeScript will allow you to use.

Setting a Compiler Target in tsconfig.json

Configuring the lib Compiler Option

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Configuring the lib Compiler Option

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