how to use auto convert of type detect in typescript when I use '=='

Question

when I use '==' to judge equality between string and number,

const a = '1234';
const b = 1234;
// This condition will always return 'false' since the types 'string' and 'number' have no overlap.
const c = a == b;

typescript show an error: This condition will always return 'false' since the types 'string' and 'number' have no overlap.

but we kown the code works in javascript;

typescript playground

so is there any way to remove the error except using Number function to convert?

Answer 1

Using the equality operator (==) to compare two values which are known to be string and number by the compiler will always result in this error. You can either use a type assertion on one of the values, or use an abstraction to do the comparison (like the function in the example below):

TS Playground

function areEqual (a: unknown, b: unknown): boolean {
  return a == b;
}

const a = '1234';
const b = 1234;
let c: boolean;

c = a == b; /*
    ~~~~~~
This condition will always return 'false' since the types 'string' and 'number' have no overlap.(2367) */

c = areEqual(a, b); // ok
c =  a as unknown == b; // ok

---

However, if the compiler doesn't know for sure that the values are string and number, you won't have a problem with the direct comparison:

TS Playground

let a = '1234' as string | number;
let b = 1234 as string | number;

const c = a == b; // ok