How to specify a min-max range for number in Typescript?

If you just want to check it at runtime:

A simpler version of @Pluto's function:

function foo(threshold: number): boolean {
    return 0 <= threshold && threshold <= 256;
}

If you want it to be type-checked:

Inspired by this blog post: https://basarat.gitbook.io/typescript/main-1/nominaltyping .

Typescript Playground link

enum SmallIntBrand { _ = "" }
type SmallInt = SmallIntBrand & number;

function isSmallInt(n: number): n is SmallInt {
    return Number.isInteger(n) &&
        0 <= n &&
        n <= 255;
}

const a = 434424;
const b = 25;

function requiresSmallInt(n: SmallInt) {
    console.log("Received number: " + n);
}

// Neither of these compile, for none of them
// have checked the variable with 'isSmallInt'.
//requiresSmallInt(a);
//requiresSmallInt(b);

if (isSmallInt(a)) {
    requiresSmallInt(a);
}

if (isSmallInt(b)) {
    requiresSmallInt(b);
}

You need to use the || operator ,If one of the conditions is incorrect ,then the condition will not enter the if :

    function foo(threshold: number):boolean {
      if (threshold < 0 || threshold > 256) {
        return false;
      }
        return true;
    }

Look at this code :

console.log(foo(-1)); // false
console.log(foo(5)); // true
console.log(foo(280)); // false

Also you can use with && operator , only if both conditions are true, then the condition will enter the if :

 function foo(threshold: number):boolean {
      if (threshold > 0 && threshold < 256) {
        return true;
      }
        return false;
    }


console.log(foo(-1)); // false
console.log(foo(5)); // true
console.log(foo(280)); // false

You wouldn't type it really. You would handle that separately:

function foo(threshold: number) {
  if (threshold < 0 || threshold > 256) {
    return;
  }
  // Whatever you want to do if threshold is OK
}