What is the best way to loop through the keys of a generic object in TypeScript?

Question

What is the best way to loop through the keys of a generic object in TypeScript?

When I come across a large object of unknown size, my usual approach is to iterate over it. In the past, I've used generators and custom Symbol.iterator functions to make these objects iterable with a for..of loop.

However, in the ever-evolving world of technology, I have found that using Object.keys in 2017 makes this process easier:

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

Surprisingly, this method works just fine. But my TypeScript compiler keeps throwing the error "error TS7017: Element implicitly has an 'any' type because type '{}' has no index signature"

Can anyone shed some light on what I might be overlooking here? Or perhaps share the current best practices for iteration using ES2015 and TypeScript (2.2.2)?

typescript ecmascript-6

Answer 1

Answer №1

Iterating with for..in

Upon examining the Typescript documentation on iterators and generators (Typescript: Iterators and Generators), we learn that the for..in syntax is utilized to loop through the keys of an object.

The distinction between for..in and for..of lies in their output - for..in returns a list of keys from the iterated object, while for..of provides values of numerical properties.

This functionality can be leveraged to effectively access specific elements within our objects:

// Using for..in to iterate over keys:
for (const key in indexedObject)
{
   // Access the item based on the current key:
   const indexedItem = indexedObject[key];
   // The desired item is now obtained.
   
   // Proceed with utilizing the item...
}

A Refined Approach

We can apply this methodology to address problems efficiently:

// Looping through keys in bigObject:
for (const key in bigObject)
{
   // Retrieve the strongly typed value associated with this key:
   const value = bigObject[key];
   // Now equipped with the strongly typed value corresponding to the key (depending on the initial typing of bigObject).
   
   // Perform noteworthy operations involving bigObject's property...
}

In acknowledgement of insights presented by jcalz (Appreciation extended):

This approach functions effectively in JavaScript; however, it may encounter TypeScript errors when employed under --strict compiler options.

Answer 2

Iterating with for..in

Upon examining the Typescript documentation on iterators and generators (Typescript: Iterators and Generators), we learn that the for..in syntax is utilized to loop through the keys of an object.

The distinction between for..in and for..of lies in their output - for..in returns a list of keys from the iterated object, while for..of provides values of numerical properties.

This functionality can be leveraged to effectively access specific elements within our objects:

// Using for..in to iterate over keys:
for (const key in indexedObject)
{
   // Access the item based on the current key:
   const indexedItem = indexedObject[key];
   // The desired item is now obtained.
   
   // Proceed with utilizing the item...
}

A Refined Approach

We can apply this methodology to address problems efficiently:

// Looping through keys in bigObject:
for (const key in bigObject)
{
   // Retrieve the strongly typed value associated with this key:
   const value = bigObject[key];
   // Now equipped with the strongly typed value corresponding to the key (depending on the initial typing of bigObject).
   
   // Perform noteworthy operations involving bigObject's property...
}

In acknowledgement of insights presented by jcalz (Appreciation extended):

This approach functions effectively in JavaScript; however, it may encounter TypeScript errors when employed under --strict compiler options.

Answer 3

Answer №2

After experimenting for a while, I discovered a solution that satisfies the TS compiler. The trick is to define the key variable outside of the for loop:

type MyObject = { 
  property1: string; 
  property2: string 
}

const myObj: MyObject = {
  property1: "foo",
  property2: "bar"
};

// Define the key outside the for loop
let key: keyof MyObject;

for(key in myObj) {
  console.log(myObj[key]); // no compilation error
}

Answer 4

After experimenting for a while, I discovered a solution that satisfies the TS compiler. The trick is to define the key variable outside of the for loop:

type MyObject = { 
  property1: string; 
  property2: string 
}

const myObj: MyObject = {
  property1: "foo",
  property2: "bar"
};

// Define the key outside the for loop
let key: keyof MyObject;

for(key in myObj) {
  console.log(myObj[key]); // no compilation error
}

Answer 5

Answer №3

An unspecified quantity of items that share the same characteristics are contained within.

Perhaps implementing a generic interface named BigObject<T> could be beneficial for organizing your data as a dictionary?

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

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

In my example, I specified the type as object in

let bigObject: BigObject<object>

. However, you can replace it with a more specific type.

Answer 6

An unspecified quantity of items that share the same characteristics are contained within.

Perhaps implementing a generic interface named BigObject<T> could be beneficial for organizing your data as a dictionary?

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

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

In my example, I specified the type as object in

let bigObject: BigObject<object>

. However, you can replace it with a more specific type.

Answer 7

Answer №4

Efficient Object Iteration in Typescript

If you're looking for a concise way to iterate through the properties of an object in Typescript without encountering common errors, here's a neat solution that doesn't require an interface or class.

export const collection = {property1: "", property2: ""};

let key: keyof typeof collection;
for (key in collection) collection[key] = key; // perform desired operation

This code snippet efficiently assigns the values of the properties based on their respective keys. It should compile smoothly with tsc.

Addressing the Question Posed by the OP

If you're dealing with a class instead of an object literal, utilize keyof followed by the class name. For objects with unknown keys as mentioned by the original poster, consider using an inline type like this:

export const collection: { [key: string]: any } = // define inline type
  { Property1: "", Property2: "" }; // value source can vary

let key: keyof typeof collection;
for (key in collection) collection[key] = key; // perform desired operation

Answer 8

Efficient Object Iteration in Typescript

If you're looking for a concise way to iterate through the properties of an object in Typescript without encountering common errors, here's a neat solution that doesn't require an interface or class.

export const collection = {property1: "", property2: ""};

let key: keyof typeof collection;
for (key in collection) collection[key] = key; // perform desired operation

This code snippet efficiently assigns the values of the properties based on their respective keys. It should compile smoothly with tsc.

Addressing the Question Posed by the OP

If you're dealing with a class instead of an object literal, utilize keyof followed by the class name. For objects with unknown keys as mentioned by the original poster, consider using an inline type like this:

export const collection: { [key: string]: any } = // define inline type
  { Property1: "", Property2: "" }; // value source can vary

let key: keyof typeof collection;
for (key in collection) collection[key] = key; // perform desired operation

Answer 9

Answer №5

Here is a potential solution:

type ExtractKey<T> = T extends Partial<Record<infer K, any>> ? K : never;
type ExtractValue<T> = T extends Partial<Record<any, infer V>> ? V : never;
/** Converts object to strongly-typed entries. */
export const convertToEntries = <T extends Partial<Record<string, any>>>(obj: T) => {
  return Object.entries(obj) as [ExtractKey<T>, ExtractValue<T>][];
};


/** Sample key types. */
export type FilterName =
  | "category"
  | "subcategory"
  | "tagId"
  | "authorId"
  | "date";
export type Filters = Partial<
  Record<FilterName, string>
>;

/** Example usage: */
for (const [name, value] of convertToEntries(filters)) {
  switch (name) {
    case "category": {
      // ...
      break;
    }
    case "subcategory": {
      // ...
      break;
    }
    ...
    default:
      throw newNonExhaustiveSwitchError(name);
  }
}

Answer 10

Here is a potential solution:

type ExtractKey<T> = T extends Partial<Record<infer K, any>> ? K : never;
type ExtractValue<T> = T extends Partial<Record<any, infer V>> ? V : never;
/** Converts object to strongly-typed entries. */
export const convertToEntries = <T extends Partial<Record<string, any>>>(obj: T) => {
  return Object.entries(obj) as [ExtractKey<T>, ExtractValue<T>][];
};


/** Sample key types. */
export type FilterName =
  | "category"
  | "subcategory"
  | "tagId"
  | "authorId"
  | "date";
export type Filters = Partial<
  Record<FilterName, string>
>;

/** Example usage: */
for (const [name, value] of convertToEntries(filters)) {
  switch (name) {
    case "category": {
      // ...
      break;
    }
    case "subcategory": {
      // ...
      break;
    }
    ...
    default:
      throw newNonExhaustiveSwitchError(name);
  }
}

What is the best way to loop through the keys of a generic object in TypeScript?

Answer №1

Iterating with for..in

A Refined Approach

Answer №2

Answer №3

Answer №4

Efficient Object Iteration in Typescript

Addressing the Question Posed by the OP

Answer №5

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