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React.js “Uncaught TypeError: X is not a function” occurs when we try to call a value that is not a function as a function, such as calling the props object instead of a function. To fix this error, use console.log to print the value being called to confirm whether it is a function.

The following is an example of how the error occurs.

import {useState} from 'react';

// 👇️ should take props object and not setCount directly
// ⛔️ Uncaught TypeError: setCount is not a function
function Child(setCount) {
  return (
    <div>
      <button onClick={() => setCount(current => current + 1)}>
        Click
      </button>
    </div>
  );
}

export default function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <Child setCount={setCount} />

      <h2>Count: {count}</h2>
    </div>
  );
}

ChildThe component should get the props object, but we named it setCount and tried to call the props object in the component.

The best way to fix the error is to log the setCount value in the Child component and make sure it is a function.

ChildsetCountComponents should accept a props object, and should access functions on the props .

import {useState} from 'react';

// 👇️ 现在组件接受 props 对象并调用 props.setCount 函数
function Child(props) {
  console.log('props obj:', props);
  return (
    <div>
      <button onClick={() => props.setCount(current => current + 1)}>
        Click
      </button>
    </div>
  );
}

export default function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <Child setCount={setCount} />

      <h2>Count: {count}</h2>
    </div>
  );
}

Alternatively, we can deconstruct the properties in the function definition.

// 👇️ 解构 setCount 属性并直接使用它
function Child({setCount}) {
  return (
    <div>
      <button onClick={() => setCount(current => current + 1)}>Click</button>
    </div>
  );
}

export default function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <Child setCount={setCount} />

      <h2>Count: {count}</h2>
    </div>
  );
}

Another common reason for “Uncaught TypeError: XXX is not a function” error in React is when we try to call a function on a value of the incorrect type.

For example, if we try to call the method on a value that is not an array pop(), an error will be generated.

const obj = {};

// ⛔️ Uncaught TypeError: obj.pop is not a function
obj.pop();

The pop() method can only be called on arrays, so trying to call it on any other type will throw an "X is not a function" error because there is no pop function on that type.

The best way to debug this is to use console.logprint_value to print the value you call the function with and make sure it is the correct type.

const obj = {};

console.log(typeof obj); // 👉️ "object"
console.log(Array.isArray(obj)); // 👉️ false

Once it is determined that the value on which the method was called is of the expected type and that the method is supported by said type, the error will be resolved.

We can also use conditionals to check if a value is of the correct type before calling a method.

const obj = {};

if (Array.isArray(obj)) {
  console.log(obj.pop());
}