Simplifying Typescript If statements

Question

i am relative new to typescript (coming from java) and i have currently the following problem:

Given: String with a length of 3, it can only contain the charts '0', '1' and '2'. Each of them are representing a different state. Lets assume here the following: '0' -> 'no', '1' -> 'yes', '2' -> 'unknown'. What is the most readable and simplified way to implement it? Currently i am just using simple if functions which are checking what state i have at every index, like:

let state: 'no' | 'yes' | 'unknown';
  if (input[1] === '0') {
    state = 'locked';
  } else if (input[1] === '1') {
    state = 'unlocked';
  } else {
    state = 'unknown';
  }

Because i am still new into Typescript i don't know if there is a better way to do that in Typescript :/

Thanks :)

You can store your state mapping as an object, an get the value you need with [], like : `let obj = { 0: 'no', 1:'yes', 2: 'undefined'}; let str = obj[input[1]]` — Lk77, Nov 30 '22 at 16:47
I am a little confused by your logic. you say "String with a length of 3" but it appears to be a sting of length 1, or an int. also, why are you using 3 different strings to represent the same thing? 'no', '0', 'locked'. seems redundant. why not 2 or 1 of those strings? — Rick, Nov 30 '22 at 16:51
that was just an example for the char at the second position — behzad k., Dec 05 '22 at 11:36

score 1 · Answer 1 · answered Nov 30 '22 at 16:50

Do you mean a switch statement?
You can throw it in a for-loop like this:

let state: 'no' | 'yes' | 'unknown';
for (let i = 0; i < input.length; i++) {
    switch(input[i]) { 
       case '0': { 
          state = 'locked';
          break; 
       } 
       case '1': { 
          state = 'unlocked';
          break; 
       } 
       default: { 
          state = 'unknown';
          break; 
       } 
    } 
}

You can read more about TS switch statements here https://www.tutorialsteacher.com/typescript/typescript-switch

score 0 · Answer 2 · answered Nov 30 '22 at 16:55

const mapping = {
//    ^?
    '0': 'locked',
    '1': 'yes',
    '2': 'unknown'
} as const;
// const mapping: {
//     readonly '0': "locked";
//     readonly '1': "yes";
//     readonly '2': "unknown";
// }

let y = mapping['0']
//  ^?
// let y: "locked"


// this generic basicaly splits the variants for different constants same as mapping[x]
function mapState<K extends keyof typeof mapping>(k: K) {
    return mapping[k]
}

let x = mapState('1')
//  ^?
// let x: "yes"

let a: '1' | '2' = Math.random() < 0.5 ? '1' : '2'
//  ^?
// let a: "1" | "2"
let z1 = mapping[a]
//  ^?
// let z1: "yes" | "unknown"
let z2 = mapState(a)
//  ^?
// let z2: "yes" | "unknown"

Playground

If you are interested how far can that used, there is document.createElement function which returns HTMLImageElement from document.createElement('img') and around 50 other types for other args

Simplifying Typescript If statements

2 Answers2