Lets get typing JaveScript

Here's a little story about JS

import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.transferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.transferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError(); // Error 💥
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.transferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError(); // Error 💥
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => console.log(res.transferFee)); // Log: undefined
  }, [])
  
  const handleTransfer = () => {
    console.log(transferAmount, transferFee); // Log: 399, undefined
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError(); // Error 💥
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => console.log(res.transferFee)); // Log: undefined
  }, [])
  
  const handleTransfer = () => {
    console.log(transferAmount, transferFee); // Log: 399, undefined
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError(); // Error 💥
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}

So its time to talk with backend guys

import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => console.log(res)); // Log: { Currency: "DKK", TransferFee: 4.00 } ✅
  }, [])
  
  const handleTransfer = () => {
    console.log(transferAmount, transferFee); // Log: 399, undefined
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError(); // Error 💥
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete); // 💥 { amountWithFee: "3994.00" }
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete); // ✅ { amountWithFee: "403.00" }
    else
      onError();
  }
  
  const handleChange = (value) => setTransferAmount(parseFloat(value));
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={handleChange} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete); // ✅ { amountWithFee: "403.00" }
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} type="number" />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


const TransferInput = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState(null);
  const [transferAmount, setTransferAmount] = useState(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TrasferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete); // ✅ { amountWithFee: 403.00 }
    else
      onError();
  }
  
  const handleChange = (value) => setTransferAmount(parseFloat(value));
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={handleChange} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}

TypeScript

TypeScript

What

Why

How

TypeScript

  • Typed superset of JavaScript
     
  • Compiles to plain JavaScript
     
  • C# inspired syntax & type system
     
  • Made by Microsoft

Why

Why

  • Static type checking
     
  • Refactoring tools
     
  • Improved DX
     
  • Easier onboardimg
     
  • Better collaboration across areas

How

import { getTransferFee, transferMoneyNow } from 'awesome-apis';


interface ITransferInputProps {
  onComplete: () => void;
  onError: () => void;
}
const TransferInput: React.FunctionComponent<ITransferInputProps> = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState<number>(null);
  const [transferAmount, setTransferAmount] = useState<number>(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.transferFee));
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} />
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


interface ITransferInputProps {
  onComplete: () => void;
  onError: () => void;
}
const TransferInput: React.FunctionComponent<ITransferInputProps> = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState<number>(null);
  const [transferAmount, setTransferAmount] = useState<number>(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.transferFee)); // Property 'transferFee' does not exist on type IGetTransferFeeResponse. Did you mean 'TransferFee'?
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete); // Type error: string is not assignable to number
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} /> // Type error: string is not assignable to number 
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}
import { getTransferFee, transferMoneyNow } from 'awesome-apis';


interface ITransferInputProps {
  onComplete: () => void;
  onError: () => void;
}
const TransferInput: React.FunctionComponent<ITransferInputProps> = ({onComplete, onError}) => {
  
  const [transferFee, setTransferFee] = useState<number>(null);
  const [transferAmount, setTransferAmount] = useState<number>(null);
  
  React.userEffect(() => {
    getTransferFee().then(res => setTransferFee(res.TransferFee)); 
  }, [])
  
  const handleTransfer = () => {
    if(transferAmount && transferFee)
    	transferMoneyNow({ amountWithFee: transferAmount + transferFee }).then(onComplete);
    else
      onError();
  }
  
  return (
    <TransferView>
    	<Input value={transferAmount} onChange={setTransferAmount} type="number" /> 
    	<Button text="Transfer" onPress={handleTransfer} />
    </TransferView>
  );

}

Whaaat?

Whaaat?

How did type="number" fix the type error?

interface IInputProps {
  value: string;
  onChange: (value: string) => void;
}
const Input: React.FunctionComponent<IInputProps> = (props) => {
  return (
    <FabricInput style="denim" value={props.value} onChange={props.onChange}/>
  );
}

interface IInputProps {
  type: "text" | "number";
  value: string | number;
  onChange: (value: string | number) => void;
}
const Input: React.FunctionComponent<IInputProps> = (props) => {
  return (
    <FabricInput style="denim" value={props.value} onChange={props.onChange}/>
  );
}

type AllowedTypes = "text" | "number";
type TypeToValueType<Type extends AllowedTypes> = 
	Type extends "text" 
		? string
		: Type extends "number" 
			? number 
			: string | number;

interface IInputProps<Type extends AllowedTypes = "text"> {
  type?: Type;
  value: TypeToValueType<Type>;
  onChange: (value: TypeToValueType<Type>) => void;
}
const Input = <Type extends AllowedTypes>(props: IInputProps<Type>) => {
  return (
    <FabricInput style="denim" value={props.value} onChange={props.onChange}/>
  );
}

type AllowedTypes = "text" | "number";
type TypeToValueType<Type extends AllowedTypes> = 
	Type extends "text" 
		? string
		: Type extends "number" 
			? number 
			: string | number;

interface IInputProps<Type extends AllowedTypes = "text"> {
  type?: Type;
  value: TypeToValueType<Type>;
  onChange: (value: TypeToValueType<Type>) => void;
}
const Input = <Type extends AllowedTypes>(props: IInputProps<Type>) => {
  return (
    <FabricInput style="denim" value={props.value} onChange={props.onChange}/> // 💥
  );
}

type AllowedTypes = "text" | "number";
type TypeToValueType<Type extends AllowedTypes> = 
	Type extends "text" 
		? string
		: Type extends "number" 
			? number 
			: string | number;

interface IInputProps<Type extends AllowedTypes = "text"> {
  type?: Type;
  value: TypeToValueType<Type>;
  onChange: (value: TypeToValueType<Type>) => void;
}
const Input = <Type extends AllowedTypes>(props: IInputProps<Type>) => {
  return (
    <FabricInput style="denim" value={props.value} onChange={props.onChange}/> // 💥
  );
}

type AllowedTypes = "text" | "number";
type TypeToValueType<Type extends AllowedTypes> = 
	Type extends "text" 
		? string
		: Type extends "number" 
			? number 
			: string | number;

interface IInputProps<Type extends AllowedTypes = "text"> {
  type?: Type;
  value: TypeToValueType<Type>;
  onChange: (value: TypeToValueType<Type>) => void;
}
const Input = <Type extends AllowedTypes>(props: IInputProps<Type>) => {
  
  const getValue = (value: string | number) => {
    return props.type === "number" ? props.value.toString() : props.value;
  }
  
  const handleChange = (value: string) => {
    props.onChange(props.type === "number" ? parseFloat(value) : props.value);
  }
  
  return (
    <FabricInput style="denim" value={getValue(props.value)} onChange={handleChange}/>
  );
}

// Infer types
const pickItem = <List extends any[], Index extends number>(list: List, index: Index): List[Index]  => {
  return list[index];
}

const a = pickItem(["a", 1], 1); // a: string | number;
// Infer types
const pickItem = <List extends any[], Index extends number>(list: List, index: Index): List[Index]  => {
  return list[index];
}

const a = pickItem(["a", 1], 1); // a: string | number;

const pickItem = <List extends ReadonlyArray<any>, Index extends number>(list: List, index: Index): List[Index]  => {
  return list[index];
}

const a = pickItem(["a", 1] as const, 1); // a: number;
// Nullish Coalescing & Optional Chaining
interface IUser {
  email: 'john@doe.com'
  username: 'JohnDoe88',
  info?: {
    firstName: string;
    lastName: string;
    balance?: number 
  }
}

interface IBalanceInfoProps {
  user?: IUser;
}

const BalanceInfo = (props: IBalanceInfoProps) => {
  const balance = user && user.info && user.info.balance;
  
  return (
    <Text>{balance ? balance : "Not available"}</Text>
  );
};
// Nullish Coalescing & Optional Chaining
interface IUser {
  email: 'john@doe.com'
  username: 'JohnDoe88',
  info?: {
    firstName: string;
    lastName: string;
    balance?: number 
  }
}

interface IBalanceInfoProps {
  user?: IUser;
}

const BalanceInfo = (props: IBalanceInfoProps) => {
  const balance = user && user.info && user.info.balance;
  
  return (
    <Text>{balance ? balance : "Not available"}</Text>
  );
};

// 💥 Shows "Not available" instead of 0...
// Nullish Coalescing & Optional Chaining
interface IUser {
  email: 'john@doe.com'
  username: 'JohnDoe88',
  info?: {
    firstName: string;
    lastName: string;
    balance?: number 
  }
}

interface IBalanceInfoProps {
  user?: IUser;
}

const BalanceInfo = (props: IBalanceInfoProps) => {
  const balance = user?.info?.balance;
  
  return (
    <Text>{balance ?? "Not available"}</Text>
  );
};

// ✅ Shows 0 instead of "Not available"
// Discriminated Unions
type Shape = {
    kind: "square" | "rectangle" | "circle";
    size?: number;
    width?: number;
    height?: number;
    radius?: number;
}

interface IAwesomeShapeProps<S extends Shape> {
  shape: S;
}

const AwesomeShape = <S extends Shape>(props: IAwesomeShapeProps<S>) => {
  if (props.shape.kind === "square")
    return <Square size={props.shape.size || 0} />;
    
  if (props.shape.kind === "rectangle")
    return <Rectangle width={props.shape.size || 0} height={props.shape.size || 0} />;
    
  if (props.shape.kind === "circle")
    return <Circle radius={props.shape.radius || 0} />;
    
    
};
// Discriminated Unions
type Shape = {
    kind: "square" | "rectangle" | "circle";
    size?: number;
    width?: number;
    height?: number;
    radius?: number;
}

interface IAwesomeShapeProps<S extends Shape> {
  shape: S;
}

const AwesomeShape = <S extends Shape>(props: IAwesomeShapeProps<S>) => {
  if (props.shape.kind === "square")
    return <Square size={props.shape.size || 0} />;
    
  if (props.shape.kind === "rectangle")
    return <Rectangle width={props.shape.size || 0} height={props.shape.size || 0} />;
    
  if (props.shape.kind === "circle")
    return <Circle radius={props.shape.radius || 0} />;
    
    return <Text>{`Missing props for kind ${props.shape.kind}`}</Text>;
    
};
// Discriminated Unions
interface ISquare {
    kind: "square";
    size: number;
}
interface IRectangle {
    kind: "rectangle";
    width: number;
    height: number;
}
interface ICircle {
    kind: "circle";
    radius: number;
}

type Shape = Square | Rectangle | Circle;

interface IAwesomeShapeProps<S extends Shape> {
  shape: S;
}

const AwesomeShape = <S extends Shape>(props: IAwesomeShapeProps<S>) => {
  if (props.shape.kind === "square")
    return <Square size={props.shape.size} />;
    
  if (props.shape.kind === "rectangle")
    return <Rectangle width={props.shape.size} height={props.shape.size} />;
    
  if (props.shape.kind === "circle")
    return <Circle radius={props.shape.radius} />;
};

Summary

Basic Types

Interfaces

Generics

Generics

Conditional Types

Summary

Optional chaining

Nullish Coalescing

Generics

Union Types

Conditional Types

Summary

Basic Types

Interfaces

Generics

Union Types

Conditional Types

Summary

Summary

Nice to knows

Nice to knows

utility-types

utility

immer-reducer

state system

80% devs wants TS

state of JS

Use strict mode

tip

async, readonly...

new features

better error messages

TS 3.7

Nice to knows

utility-types

utility

immer-reducer

state system

80% devs wants TS

state of JS

Use strict mode

tip

async, readonly...

new features

better error messages

TS 3.7

Have a great day 🥳

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