typescript check dynamically if string is a property of complex object

Question

I really need to be able to check dynamically if aa is a property of my complex object called validations

const check = 'propertya' + 'a'; // result of a complex calculation that returns string 'propertyaa'

validations= {
    'a' : {
      'propertyaa': 'ax',
      'ab': 'ay',
      'ac': 'az'
    },
    'b' : {
      'ba': 'ax',
      'bb': 'ay',
      'bc': 'az'
    }
  };

if (this.validations.a[check] === undefined) {  ...

Error is:

element implicitly has an any type because type '{ ' propertyaa': string, 'ab': string, 'ac': string; }' has no index signature
(property) 'a': {
    'propertyaa': string;
    'ab': string;
    'ac': string;
}

Curiously the static (not dynamic) variant works if (this.validations.a['ab']) {

This bit gives you the error: this.validations.a[check]? I can't reproduce this. — pushkin
– pushkin, Commented May 2, 2018 at 14:30

Titian Cernicova-Dragomir · Accepted Answer · 2018-05-02 14:40:54Z

You can go one of two routes. You can either add an index signature to your validation properties:

type Indexable = { [name: string]: string }
const validations = {
    'a': {
        'propertyaa': 'ax',
        'ab': 'ay',
        'ac': 'az'
    } as Indexable,
    'b': {
        'ba': 'ax',
        'bb': 'ay',
        'bc': 'az'
    } as Indexable
};

const check = 'propertya' + 'a';
if (validations.a[check] === undefined) {
}

You could also just cast the property to Indexable when you use it:

if ((validations.a as Indexable)[check] === undefined) {
}

You could also use any instead of Indexable, but the Indexable defined above provides more type safety then any

Another option is to assert that the string check is in fact a key for the value of a:

if (validations.a[check as 'propertyaa'] === undefined) {
}

Edit

If you go with the first option, a helper function can help avoid the need for the type assertion on each property of validations:

type Indexable = { [name: string]: string }
const validations = createValidations({
    'a': {
        'propertyaa': 'ax',
        'ab': '19',
        'ac': 'az'
    },
    'b': {
        'ba': 'ax',
        'bb': 'ay',
        'bc': 'az'
    }
});
function createValidations<T>(o: { [P in keyof T]: T[P] & Indexable}) : typeof o {
    return o;
}
const check = 'propertya' + 'a';
if (validations.a[check] === undefined) {
}

Matěj Pokorný · Accepted Answer · 2018-05-02 14:34:04Z

0

You will need index signature...

interface ComplexObject {
    [index: string ]: string | ComplexObject;
}

const check = 'propertya' + 'a';

const validations: ComplexObject = {
    'a' : {
      'propertyaa': 'ax',
      'ab': 'ay',
      'ac': 'az'
    },
    'b' : {
      'ba': 'ax',
      'bb': 'ay',
      'bc': 'az'
    }
  };

if (this.validations.a[check] === undefined) {
    // ...
}

answered May 2, 2018 at 14:34

Matěj Pokorný

18.1k6 gold badges43 silver badges50 bronze badges

1 Comment

annepic Over a year ago

Sorry but I get: Element explicitly has an any type because type string | ComplexObject has no index signature. Am i missing something? thank u! Now i get this even if i check for this.validations.a['propertyaa']

annepic · Accepted Answer · 2018-05-04 11:22:45Z

0

There is a simpler solution and really the only one that works for me so far:

cartoons = {
      'producers': {
          'Disney': 'Walt Disney',
          'PIXAR1': 'John John'
       }
    }

const check = 'Dis' + 'ney';
    const y = (<any>this.cartoons.producers)[check]; // notice the cast to any
    alert(y);

answered May 4, 2018 at 11:22

annepic

1,4273 gold badges16 silver badges24 bronze badges

Collectives™ on Stack Overflow

typescript check dynamically if string is a property of complex object

3 Answers 3

Comments

1 Comment

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Related