Category: JavaScript

Sometimes, we want to fix the "Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type" error with TypeScript.

In this article, we’ll look at how to fix the "Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type" error with TypeScript.

How to fix the "Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type" error with TypeScript?

To fix the "Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type" error with TypeScript, we should declare our variable with a type annotation.

For instance, we write

const color: { [key: string]: any } = {
  red: null,
  green: null,
  blue: null,
};

to declare the color object with the { [key: string]: any }, which lets us include any properties with string keys in the color object.

Conclusion

To fix the "Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type" error with TypeScript, we should declare our variable with a type annotation.

Sometimes, we want to fix the "Cannot read property ‘push’ of undefined in [null]" error with TypeScript.

In this article, we’ll look at how to fix the "Cannot read property ‘push’ of undefined in [null]" error with TypeScript.

How to fix the "Cannot read property ‘push’ of undefined in [null]" error with TypeScript?

To fix the "Cannot read property ‘push’ of undefined in [null]" error with TypeScript, we need to make sure the variable we call push on is an array.

For instance, we write

const stringArr: Array<string> = [];

or

const stringArr: string[] = [];

to declare the stringArr array.

Then we can call stringArr.push without errors.

Conclusion

To fix the "Cannot read property ‘push’ of undefined in [null]" error with TypeScript, we need to make sure the variable we call push on is an array.

Sometimes, we want to fix TypeScript ts-jest not recognizing ES6 imports.

In this article, we’ll look at how to fix TypeScript ts-jest not recognizing ES6 imports.

How to fix TypeScript ts-jest not recognizing ES6 imports?

To fix TypeScript ts-jest not recognizing ES6 imports, we need to transform ES6 modules into something Jest can use.

For instance, we write

{
  "jest": {
    "transform": {
      "^.+\\.jsx?$": "babel-jest",
      "^.+\\.tsx?$": "ts-jest"
    }
    // ...
  }
}

in our Jest config file to transform jsx files with babel-jest so they can be run with

"^.+\\.jsx?$": "babel-jest"

Conclusion

To fix TypeScript ts-jest not recognizing ES6 imports, we need to transform ES6 modules into something Jest can use.

Sometimes, we want to declare third party modules with TypeScript.

In this article, we’ll look at how to declare third party modules with TypeScript.

How to declare third party modules with TypeScript?

To declare third party modules with TypeScript, we use the declare keyword.

For instance, we write

declare module "foo-module" {
  function foo(): void;
  export = foo;
}

in a .d.ts file in our TypeScript project to declare the foo-module module and its members.

Then we can import the module without errors by writing

import * as foo from "foo-module";
foo();

in our code.

Conclusion

To declare third party modules with TypeScript, we use the declare keyword.

To fix the error ‘Component cannot be used as a JSX component. Its return type ‘Element[]’ is not a valid JSX element’ with React TypeScript, we need to make sure our component returns a single root element.

For instance, we write

function Todos(): JSX.Element {
  return (
    <>
      {todos.map((todo) => (
        <li>{todo.task}</li>
      ))}
    </>
  );
}

to put our array of items in a fragment so that the component can be compiled.

We call todos.map to render an array of li elements so we need to wrap them with a fragment.