Allow inline type predicates

[saschanaz]

From #5731

if (<foo is Element>(foo.nodeType === 1)) { 
    // Assume foo is Element here
}

A proposal from @sandersn: #5731 (comment)

Type Predicate Expressions

A type predicate expression allows you to narrow a union type with a single expression. The syntax is

<variable is type> boolean-expression

When the expression is used in a conditional context, the true branch of the conditional narrows variable to type, while the false branch narrows variable by removing type.

The type predicate expression is of type Boolean, with apparent type of type predicate. The right hand side must be an expression of type Boolean. The left hand side's variable must be a name bound in the current scope. The left hand side's type must be a name bound in the current scope.

Open questions

1. How does variable capture work? Can a type predicate expression be returned from a function and used to narrow a variable that's no longer in scope? This could be defined to cause an error, but that restriction is neither obvious nor easy to use.
2. Why reuse the type assertion syntax? The type of the right expression is checked to be Boolean, unlike assertions, and the resulting type is still Boolean, also unlike assertions.

[pushkine]

Here's a workaround that gets removed by bundlers

const Narrow = <T>(v): v is T => true;

if (foo.nodeType === 1 && Narrow<Element>(foo)) {
}

(edit 2022) Here's another workaround that works as a statement (= works in switch blocks)

function Narrow<T extends R, R = unknown>(value: R): asserts value is T {}

switch (foo.nodeType) {
	case 1: {
		foo; // any
		Narrow<Element>(foo);
		foo; // Element
	}
}

Playground Link

[jsejcksn]

Here's a workaround that gets removed by bundlers

const Narrow = <T>(v): v is T => true;

if (foo.nodeType === 1 && Narrow<Element>(foo)) {
}

Combined:

playground link

function narrow <T>(value: unknown, expr: unknown): value is T {
  return Boolean(expr);
}

declare const foo: Node;

if (narrow<Element>(foo, foo.nodeType === 1)) {
  foo; // Element
}

[jsejcksn]

if (<foo is Element>(foo.nodeType === 1)) { 
    // Assume foo is Element here
}

@saschanaz What would it look like using as syntax? Something like this?

if (foo.nodeType === 1 as foo is Element) {
  foo; // Element
}

[saschanaz]

It should probably still be with parens - (foo.nodeType === 1), but basically I'd expect such form.

[steinybot]

Here is another workaround:

function isF<A, B extends A>(f: (a: A) => boolean): (a: A) => a is B {
  return ((a: A) => f(a)) as (a: A) => a is B
}

function filterUndefined<A>(as: (A | undefined)[]): A[] {
  return as.filter(isF((a) => a !== undefined))
}

[jsejcksn]

Functions have only one output but have multiple inputs and type guards only address one input. We can fix this by allowing type guards to address multiple inputs:

function isPair(foo: Node, bar: Node): foo is Element, bar is Text;

if (isPair(foo, bar)) {
    // Assume foo is Element here
    // Assume bar is Text here
}

+1 for proper compound predicates

Aside: You can hack this now by packing the candidates into a tuple (and then unpacking):

Playground

declare function isPair (tuple: readonly [Node, Node]): tuple is [Element, Text];

declare const foo: Node;
declare const bar: Node;

const fooBarPack = [foo, bar] as const;

if (isPair(fooBarPack)) {
  const [
    foo,
  //^? const foo: Element
    bar,
  //^? const bar: Text
  ] = fooBarPack;
}

Of course (because it's a hack) it requires (wastefully) creating that tuple.

[ethanresnick]

You could also imagine doing this as the annotation for a variable holding the predicate's result, i.e.:

declare const foo: Node;
const isElement: foo is Element = foo.nodeType === 1;

if (isElement) {
    // Assume foo is Element here
}

[brandonmcconnell]

The ideal API for this in my opinion would be something similar to the way that type guard functions assert types, but allowed in more places, like on if-statements. For example:

Before — requires separate function for type guarding

function isIterable<T>(obj: Iterable<T> | ArrayLike<T>): obj is Iterable<T> {
  return typeof (obj as Iterable<T>)[Symbol.iterator] === 'function';
}

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  if (isIterable(obj)) {
    // some logic here
  } else {
    // some logic here
  }
}

After — can handle type guarding inline via if statement

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  const isIterable = typeof (obj as Iterable<T>)[Symbol.iterator] === 'function';
  if (isIterable): obj is Iterable<T> {
    // some logic here
  } else {
    // some logic here
  }
}

** this could just as easily be rewritten to get rid of the isIterable placeholder altogether, like this:

function iterateOver<T>(obj: Iterable<T> | ArrayLike<T>, expose: Function) {
  if (typeof (obj as Iterable<T>)[Symbol.iterator] === 'function'): obj is Iterable<T> {
    // some logic here
  } else {
    // some logic here
  }
}