We can use the TypeError() constructor function to throw or show an error when the data type doesn't match in JavaScript.

Let's consider a function which accepts values of string type only and wants to throw an error on an invalid type.

So let's create that function,

// a function which accepts only
// values of string type
function iAcceptStringOnly(name) {
  // cool code goes here!
}

Now, To know what is the type of data, we can use the typeof operator. This operator will return the type as a string eg: string, number, boolean, etc,.

// a function which accepts only
// values of string type
function iAcceptStringOnly(name) {
  // get what is the type of the parameter
  const typeOfParameter = typeof name;
}

Now check if the name variable is of type string using an if statement. If it is not of string type then it will throw a TypeError 🦄.

// a function which accepts only
// values of string type
function iAcceptStringOnly(name) {
  // get what is the type of the parameter
  const typeOfParameter = typeof name;

  // check whether the name variable has type string
  if (typeOfParameter != "string") {
    throw new TypeError();
  }
}

The TypeError() constructor functions accepts:

• the error name as the first parameter
• the filename where the error occurred as the second parameter
• the line number where the error occurred as the third parameter

These parameters are optional, but it's good to define them for understanding the error.

// a function which accepts only
// values of string type
function iAcceptStringOnly(name) {
  // get what is the type of the parameter
  const typeOfParameter = typeof name;

  // check whether the name variable has type string
  if (typeOfParameter != "string") {
    throw new TypeError("the name should be of string type", "main.js", 5);
  }

  console.log("No Error!, it's cool 🧘‍♂️");
}

Now try invoking the function iAcceptStringOnly with a number and observe the error shown.

See this example live in JSBin.

Feel free to share if you found this useful 😃.

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