This expression is not callable. Type 'String' has no call signatures

This expression is not callable. Type String has no call signatures #

The "This expression is not callable. Type 'String' has no call signatures" TypeScript error occurs when we try to call a string as a function or have typed a function as a string. To solve the error, make sure you're calling a function and it is typed as a function.

Here is an example of how the error occurs.

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const str = 'hello world';

// ⛔️ This expression is not callable.
// Type 'String' has no call signatures.ts(2349)
str();

The example above shows how trying to call a string as a function causes the error. The same is the case with any other type that is not callable.

You could be forgetting to access a built-in method on the string.

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const str = 'hello world';

console.log(str.toUpperCase()); // 👉️ "HELLO WORLD"

Make sure to access any built-in methods as str.someMethod(), and not str().

If you console.log the value and it is a function, then maybe your typings are incorrect.

Hover over the function you're trying to invoke and make sure it has a type of function.

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function getFunc(): any {
  return (a: number, b: number) => {
    return a + b;
  };
}

// 👇️ const result: string
const result = getFunc() as string;

// ⛔️ Error: This expression is not callable.
// Type 'String' has no call signatures.ts(2349)
result(50, 50);

The example above shows how somehow the function got assigned a different type (a string) somewhere in the codebase (most commonly caused by using type assertions).

Here are some examples of how to type a function using a type alias or an interface.

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// 👇️ using Type Alias (for only function) 👇️
type FunctionA = (a: number, b: number) => number;
const funcA: FunctionA = (a, b) => a + b;
console.log(funcA(25, 25));

// 👇️ Using interface (for only function) 👇️
interface FunctionB {
  (a: number, b: number): number;
}
const funcB: FunctionB = (a, b) => a + b;
console.log(funcB(30, 30));

// 👇️ Type Alias with other members 👇️
type DateThings = {
  date: string;
  getTimestamp: (str: string) => number;
};
const d: DateThings = {
  date: '2023-09-24',
  getTimestamp(str) {
    return new Date(str).getTime();
  },
};
console.log(d.getTimestamp('2024-04-16'));

// 👇️ Interface with other members 👇️
interface MathThings {
  sum: (a: number, b: number) => number;
  num: number;
}
const obj: MathThings = {
  num: 100,
  sum(a, b) {
    return a + b;
  },
};
console.log(obj.sum(45, 45));

The first two examples show how you would correctly type a function using a type alias or an interface that only represents a single function.

Conclusion #

To solve the "This expression is not callable. Type 'String' has no call signatures" TypeScript error:

1. Make sure the value you are calling as a function is an actual function.
2. Make sure the function is typed correctly.
3. TypeScript won't let you call a function that is typed as something else.