How to iterate over keys of a generic object in TypeScript?

Question

I need to iterate over a large object which is just typed as "object". It contains an unknown number of objects of the same type.

In older posts I had found solutions using a generator within a custom Symbol.iterator function to make the large object iterable with a for..of loop.

But it seems to me, now in 2017, just using Object.keys is actually easier:

Object.keys(bigObject).forEach((key:string)=>{
console.log(bigObject[key]);
});

This actually runs just fine! But the TypeScript compiler keeps giving me the error "error TS7017: Element implicitly h as an 'any' type because type '{}' has no index signature"

Does anybody have an idea what I am missing here? Or what is the current best practice to do such an iteration with ES2015 and TypeScript (2.2.2)?

Solution 1

It contains an unknown number of objects of the same type.

Maybe with a generic interface BigObject<T> for your dictionary?

interface BigObject<T> {
    [index: string]: T
}

let bigObject: BigObject<object> = {}
Object.keys(bigObject).forEach(key => {
  console.log(bigObject[key])
})

Here I wrote the type object in let bigObject: BigObject<object>. You can use a better type.

Solution 2

for..in

When looking at the Typescript documentation (Typescript: Iterators and Generators), we see that the for..in syntax will iterate over the keys of the object.

for..in returns a list of keys on the object being iterated, whereas for..of returns a list of values of the numeric properties of the object being iterated.

We can use that to our advantage to index into our object and get the strongly typed value:

// Go through each key of the indexed object:
for (const key in indexedObject)
{
   // Get the indexed item by the key:
   const indexedItem = indexedObject[key];
   // Now we have the item.

   // Use it...
}

Solution

We can use that to get an elegant solution to the question:

// Go through each key in the bigObject:
for (const key in bigObject)
{
   // Get the strongly typed value with this name:
   const value = bigObject[key];
   // Now we have the the strongly typed value for this key (depending on how bigObject was typed in the first place).

   // Do something interesting with the property of bigObject...
}

Comments

[Aleksey L.]

You can "type" your object as any or { [index:string] : any }

[Marcel]

thanks a lot for the quick answer, Aleksey! Yeah, that gets rid of the error. But I am just wondering if this entire approach is what is considered best practice these days? Also with the "object" type just being introduced with TypeScript 2.2.2 I thought this was the way to go when dealing with an object of unknown type...

[Marcel]

thanks! :-) So it seems, the combination of my above approach with the definition of an indexable type (interface that defines index signature) is the way to go then...

[Paleo]

@Marcel Yes. You could just use an inline type definition: let bigObject: {[index: string]: object} = {}. It is the proposition of @Aleksey L., but with object instead of any.

[Marcel]

@Paleo...yeah, true. in my case I am going to stick to an interface since I have multiple similar objects which will be processed together. thanks again!

[Marcel]

just as an addition for anyone trying to do this as well: with TypeScript 2.2.2 the handling of string indexing for objects with index signatures has been simplified. so, now instead of writing bigObjectt[key] in the above examples, you can just write bigObject.key ! :-) here is a link to the relevant TypeScript blog post ... link

[Paleo]

@Marcel: Nope. If key is a variable that contains abc, bigObject.key will not access to bigObject.abc. You have to keep the syntax bigObject[key] here. By the way, can you validate my answer?