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Things you might not know about Typescript

Asim Dahal
Developer with a special skill of being lazy
・3 min read

Things you might not know about Typescript

Typescript is an awesome language. I have been using it for a year now and my love towards the language is infinity. It's commonly known as the superset of Javascript, but it's much more than that. Typescript helps a lot during the day to day development work, also it helps a lot to avoid the unnecessary painful bugs.

Here, in this articles I am going to present some features ina Typescript that you might not know and that may come handy in your day to day life. Are you excited to go through the features?

@ts-expect-error

Let's assume a case where the ts compiling is yelling at you that you are doing something wrong,

const multiply = (a:number,b:number) => a+b;
multiply('12',13);
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Here, TS compiler yells at you with the message
Argument of type 'string' is not assignable to parameter of type

Let's say you cannot change the type of the first argument and temporarily want to surpress the error being shown by the TS compiler, normally we use @ts-ignore to surpress the error

const multiply = (a:number,b:number) => a*b;
// @ts-ignore
multiply('12',13);
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Now, after some time, you fixed the error and changed the first argument of the multiply function from '12' to 12

const multiply = (a:number,b:number) => a+b;
// @ts-ignore
multiply(12,13);
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but, you forgot to remove the @ts-ignore directive we used previously, so basically it will keep ignoring the next line forever unless you remove it which may cause bug in the future.

So in this case, we can make use of @ts-expect-error directive, which does the same job as the @ts-ignore directive but as soon as the error is fixed, the TS compiler yells at you.

const multiply = (a:number,b:number) => a+b;
// @ts-expect-error
multiply(12,13);
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Unused '@ts-expect-error' directive.

This reminds you to remove the directive as soon as the error is fixed.

never type

You have a function that takes an error status code and always throws an error according to the status, the never type comes handy when you know a function is never going to reach it's end point.

The difference between never and void is, while void means atleast undefined or null is being returned, never means the function's end point is never reached.

function throwError(error: string): never { 
        throw new Error(error); 
} 
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Template Literal Types

Template literal types are similar to string litreral types in javascript but are specific to types. Let's say you have a library that implements a popover and there is a type for positioning the popover

type popoverPositions = 'top'|'bottom'|'left'|'right'|'top-left'|'top-right'|'top-center'|'bottom-left'|'bottom-right'|'bottom-center';
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It can be hectic to combine all the possible permutations and combinations for all the types.

Using template literal types, you can separate the directions and combine the types to get a new type that consists of all the possible combinations

type positions = 'top'|'bottom'|'center';
type directions = 'left'|'right'|'center'
type popoverPositions = positions | directions | `${positions}-${directions}`
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which will generate all the types as

type popoverPositions = positions | directions | "top-left" | "top-right" | "bottom-left" | "bottom-right" | "center-left" | "center-right"
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Null assertions

Null assertions basically tells your TS compiler that your value is nor null neither undefined. Let's say you have initialized the value as
let myNumber:null | number = null;
but later on you update the value of myNumber
myNumber = 69;

Now, lets assume you have a function that accepts only numbers,

const add = (a:number,b:number) => {
    return a + b;
}
add(myNumber,1);
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The compiler yells at you saying
Argument of type 'null' is not assignable to parameter of type 'number'.

So here, you can make use of null assertions with bang ! in the end of the variable to tell the compiler that the value we are sending is not null.

const add = (a:number,b:number) => {
    return a + b;
}
add(myNumber!,1);
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The code above compiles successfully.

Merging Interfaces

Megring Interfaces is the type of declaration merging, when you have two interfeces with the same name, it merges and creates a single interface

interface Box {
  height: number;
  width: number;
}
interface Box {
  scale: number;
}
let box: Box = { height: 5, width: 6, scale: 10 };
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So here, we can create two separate interfaces with the same name which in turn gets merged to a single one and can be used as per the example mentioned above.

I hope, you learnt something new from the list above.

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Discussion (1)

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captainyossarian profile image
yossarian

Btw, one more thing, you probably did not know: never type represents an empty union :)