[] : {} syntax in TypeScript

Question

I have following source code:

const positions = {
  [modules.module1.tasks.t1.id]: { modifier: 1 },
};

Anybody can explain or link to documentation what is done above?

{ modifier: 1 } is just a normal object as you would expect. { [foo]: bar } is just creating a new object where foo a variable string (unlike modifier). An alternative for the former is { ['modifier']: 1 } -- developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Caramiriel
– Caramiriel, Commented Jul 9, 2020 at 11:19
So how to make minimum viable working code in: repl.it/@tomaszwaszczyk/in-TypeScript ? — Tomasz Waszczyk
– Tomasz Waszczyk, Commented Jul 9, 2020 at 11:30

Karol Majewski · Accepted Answer · 2020-07-09 11:35:10Z

3

There is nothing specific to TypeScript in your code sample. It's just modern JavaScript.

Let's decompose what is happening here:

[modules.module1.tasks.t1.id]

This is a computed property name. It means the position object will have a property equal to modules.module1.tasks.t1.id.

If modules.module1.tasks.t1.id is a string, then this property will be exactly the same. Otherwise, modules.module1.tasks.t1.id will be coerced into a string.

{ modifier: 1 }

Our dynamic property will have a value of { modifier: 1 }. It's just a regular property assignment.

Example

const modules = {
  module1: {
    tasks: {
      t1: {
        id: 'foo'
      }
    }
  }
}

const positions = {
  [modules.module1.tasks.t1.id]: { modifier: 1 },
}; // evaluates to { foo: { modifier: 1 } }

edited Jul 9, 2020 at 11:35

answered Jul 9, 2020 at 11:29

Karol Majewski

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