Typescript: Allow method type guards

[vagarenko]

TypeScript Version: 2.0.3

Code

Why I can have function type guard:

function hasValue<T>(value: T | undefined): value is T { return value !== undefined; }

but not method type guard:

export class Maybe<T> {
    constructor(public value: T | undefined) {}

    hasValue(): this.value is T { return this.value !== undefined; }
    //              ^ '{' or ';' expected.
}

?

[kitsonk]

Semi related to a random thought here in that more complicated guards could express codependency of properties:

interface Array<T> {
    length: number;
    pop(): this.length > 0 ? T : undefined;
}

const arr = [ 1, 2, 3 ];

if (arr.length > 0) {
    const val = arr.pop(); // val is number, not number | undefined under strictNullChecks
}

[aluanhaddad]

This would be interesting... expressing path dependent types.
Read the OP backward... too early.

[mattmazzola]

Would this also fix issues with Map?
In the case where you're using map to add memoization to functions there is a similar need to use the ! operator.

const cache = new Map<number, string[]>()
if (cache.has(n)) {
  return cache.get(n)!  // The if statement above should make return type `string[]` instead of `string[] | undefined`
}

[DanielRosenwasser]

What about using this type guards instead?

export class Maybe<T> {
    constructor(public value: T | undefined) {}

    hasValue(): this is this & { value: T } { return this.value !== undefined; }
}

///////

declare var x: Maybe<string>;
if (x.hasValue()) {
    x.value.toLowerCase();
}

@mattmazzola I've used the above technique to describe a way that Map#has could work similarly, but it can get a bit messy.

[mattmazzola]

Ok, I'll have to learn/explore more about the this type guards. In my Map example this type definition is pre-defined by the lib.d.ts since they're part of ES2016 I believe and I didn't want to re-write my own type which wraps the native one.

[masaeedu]

@DanielRosenwasser Any way to make that approach work for private properties?

[sgronblo]

I wanted to use this for this pattern:

https://www.typescriptlang.org/play/?ssl=14&ssc=1&pln=15&pc=1#code/KYDwDg9gTgLgBAYwDYEMDOa4BEU2ASQDs8oA3FJOAbwCg44woBLcvRCQtGKAVwRmgAKRi1zA4XFLABc2MQBoGzVuOCEAJrJxsAPnEI8kSAJTUAvjTpwmaAPKEA5hCaPBx2TAAWN65i8+AMmo4NU19Q0ozait6KGAYHihCOH80ADpQuABeHPCjKwsrGwAxFyY8Nw9vTB9UuCCqEI0tMTgo2npY+MTk1IyNOABCXIN8+kL6AVLCcuAAZW4XBzdozusAMzhBPpKyiuNTDrW4OISkuAADABIqPslYNIEF5ldjKIBaOBu+0MeIZ6WbjMFxibVBXigEAA7vpgDCAKJQSFQQQAcgAKtU5HgiCRyJQfBwnEtFAgUIRCBB4AgOKRgLAUhA4Os9uIXHiKBJFo5UcYCjQzEA

export class DateInterval {
  private constructor(private start: Date, private end: Date | null) {}

  isOngoing(): this is this & { end: null } {
    return this.end === null
  }

  isFinite(): this is this & { end: Date } {
    return this.end !== null
  }

  toFiniteString() {
    if (this.isFinite()) {
      return `${this.start.toString()} - ${this.end.toString()}`
    }
    throw new Error('This DateInterval is ongoing, cannot convert to finite interval string')
  }
}

But it gives me the following error: "Property 'end' has conflicting declarations and is inaccessible in type 'DateInterval & { end: Date; }'.(2546)"

[rodrigost23]

The problem with the solution by @DanielRosenwasser:

What about using this type guards instead?

export class Maybe<T> {
    constructor(public value: T | undefined) {}

    hasValue(): this is this & { value: T } { return this.value !== undefined; }
}

///////

declare var x: Maybe<string>;
if (x.hasValue()) {
    x.value.toLowerCase();
}

is that "else" doesn't work:

if (x.hasValue()) {
    x.value.toLowerCase();
}
else {
  x.value // x.value is string | undefined instead of undefined
}

Is there any way to negate the hasValue() function and get undefined, in this case?