What is it

is checks if a value is of a specified type and returns a boolean output

// below narrows to string
function isString(test: any): test is string{
  return typeof test === "string";
}

Syntax of variableName is variableType

// `pet is Fish`
function isFish(pet: Fish | Bird): pet is Fish {
  // returns boolean
  return (pet as Fish).swim !== undefined;
}

let pet: Fish | Bird;

if(isFish(pet)) {
  // true, pet is Fish
  pet.swim();
} else {
  // false, pet is NOT Fish
  pet.fly();
}

Any time isFish is called with some variable, TypeScript will narrow that variable to that specific type if the original type is compatible

// Both calls to 'swim' and 'fly' are now okay
let pet = getSmallPet();
 
if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}

Notice that TypeScript not only knows that pet is a Fish in the if branch; it also knows that in the else branch, you don’t have a Fish, so you must have a Bird.

You may use the type guard isFish to filter an array of Fish | Bird and obtain an array of Fish

Predicate over Boolean

Difference between using Boolean as a return type and type predicate is that when using a type predicate, the boolean return value of type predicate will be understood by TS as narrowing of type

// this type predicate function will return a boolean
function isString(test: any): test is string{
  return typeof test === "string";
}

function example(foo: any){
  // `isString` return value is not just a boolean, but understood as a `String` type enforcement
  if(isString(foo)){
    console.log("it is a string" + foo);
    console.log(foo.length); // string function is guaranteed to run as expected
  }
}