Type predicates in TypeScript help you narrowing down your types based on conditionals. They’re similar to type guards, but work on functions. They way the work is, if a function returns true, change the type of the paramter to something more useful.

Let’s start with a basic example. Let’s say you have a function that checks if a certain value is of type string:

function isString(s) {
  return typeof s === 'string';
}

Use the isString function inside another function:

function toUpperCase(x: unknown) {
  if(isString(x)) {
    x.toUpperCase(); // ⚡️ x is still of type unknown
  }
}

TypeScript throws an error. We can be sure that x is of type string at this point. But since the validation is wrapped in a function, the type of x does not change (as opposed to type guards). Enter type predicates.

Let’s tell TypeScript explicitly that if isString evaluates to true, the type of the parameter is a string:

function isString(s): s is string {
  return typeof s === 'string';
}

TypeScript now knows that we are dealing with strings in our toUpperCase function.

function toUpperCase(x: unknown) {
  if(isString(x)) {
    x.toUpperCase(); // ✅ all good, x is string
  }
}

See that in the TypeScript playground;

Narrowing down sets #

This not only helps you for unknown types, or multiple types, but also to narrow down sets within a type. Let’s have a program where you throw a dice. Every time you throw a Six, you win.

function pipsAreValid(pips: number) {
  // we check for every discrete value, as number can 
  // be something between 1 and 2 as well.
  return pips === 1 || pips === 2 || pips === 3 ||
    pips === 4 || pips === 5 || pips === 6;
}
function evalThrow(count: number) {
  if (pipsAreValid(count)) {
    // my types are lying 😢
    switch (count) {
      case 1:
      case 2:
      case 3:
      case 4:
      case 5:
        console.log('Not today');
        break;
      case 6:
        console.log('Won!');
        break;
      case 7:
        // TypeScript does not complain here, even though
        // it's impossible for count to be 7
        console.log('This does not work!');
        break;
    }
  }
}

The program looks good at first, but has some issues from a type perspective: count is of type number. This is ok as an input parameter. Right away we validate that count is a number between 1 and 6. Once we validate this, count is not any number anymore. It’s narrowed down to a discrete set of six values.

So starting from the switch statement, my types are lying! To prevent any further complications, let’s narrow down the set of numbers to those six discrete values, using union types:

type Dice = 1 | 2 | 3 | 4 | 5 | 6;
function pipsAreValid(pips: number): pips is Dice {
  return pips === 1 || pips === 2 || pips === 3 ||
    pips === 4 || pips === 5 || pips === 6;
}
function evalThrow(count: number) {
  if (pipsAreValid(count)) {
    // count is now of type Dice 😎
    switch (count) {
      case 1:
      case 2:
      case 3:
      case 4:
      case 5:
        console.log('Not today');
        break;
      case 6:
        console.log('Won!');
        break;
      case 7:
        // TypeScript errors here. 7 is not in the union type of 
        // Dice
        console.log('This does not work!');
        break;
    }
  }
}

A lot type safer for us, and for our colleagues. Of course this “type casts” can be anything that makes sense to strengthen your applications. Even if you validate complex objects, you can narrow down your parameters to a specific type and make sure they get along with the rest of your code. Useful, especially if you rely on a lot of functions.

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