In a lot of our code we have to cast string literals to literals types to conform to types so assignments work:

  type Styles = {
    position: 'relative' | 'absolute'
  }
  
  let styles = {
    position: 'relative' as 'relative'
  }
  
  let s: Styles = styles 

This is more common when using React HOCs but above example demonstrate the problem well enough.

With TypeScript 3.4 we'll have the as const literal expression which should be preferred to string or number literal types in this case because changing the value does not require changing the type.

Our example code would be better written as:

type Styles = {
  position: "relative" | "absolute";
};

let styles = {
  position: "relative" as const
};

let s: Styles = styles;

More examples:

let a: 'str' = 'str'; // bad 
let a = <'str'>'str'; // bad 
let a = 'str' as 'str' //bad 

let a = 'str' as const // good

// bad
let o = {
    foo: 'string' as 'string'
}

// good
let o = {
    foo: 'string' as const
}

Rule name

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Options

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