TypeError: startsWith is not a function in JavaScript

# TypeError: startsWith is not a function in JavaScript

The "TypeError: startsWith is not a function" error occurs when we call the startsWith() method on a value that is not a string.

To solve the error, convert the value to a string using the toString() method or make sure to only call the startsWith method on strings.

Here is an example of how the error occurs.

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const str = 123;

// ⛔️ TypeError: startsWith is not a function
const result = str.startsWith('1');

We called the String.startsWith method on a number and got the error back.

# Only call the startsWith method on strings

To solve the error, make sure to only call the startsWith() method on strings. You can convert most values to a string by using the toString() method.

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const str = 123;

const result = str.toString().startsWith('1');

console.log(result); // 👉️ true

You can also use the String() constructor to convert a value to a string.

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const str = 123;

const result = String(str).startsWith('1');

console.log(result); // 👉️ true

The String() constructor converts the supplied value to a string and returns the result.

# Check if the value is of type string before calling startsWith()

Alternatively, you can check if the value is a string before calling the startsWith() method.

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const str = null;

const result =
  typeof str === 'string' ? str.startsWith('a') : false;

console.log(result); // 👉️ false

We used the ternary operator to check if the str variable stores a string.

If it does, the value to the left of the comma is returned, otherwise, the value to the right is returned.

You can also use a simple if statement to achieve the same result.

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const str = null;

let result = false;

if (typeof str === 'string') {
  result = str.startsWith('a');
}

console.log(result); // 👉️ false

If the error persists, console.log the value you're calling the startsWith method on and check its type using the typeof operator, e.g. console.log(typeof myStr).

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console.log(typeof 'bobbyhadz.com'); // 👉️ string
console.log(typeof {}); // 👉️ object
console.log(typeof 123); // 👉️ number

If the value is an object, there's a very good chance that you are forgetting to access a specific property on which you need to call the startsWith() method.

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// ✅ with objects
const obj = {
  name: 'bobby',
};

const result1 = obj.name.startsWith('bo');
console.log(result1); // 👉️ true

// -----------------------------------------
// ✅ with arrays

const arr = ['bobby', 'hadz', 'com'];

const result2 = arr[0].startsWith('bo');
console.log(result2); // 👉️ true

We accessed a property on the object and an element in the array before calling the startsWith() method.

# Check if all strings in an array start with a specific substring

If you want to check if all strings in an array start with a specific substring, use the every() method to iterate over the array and then call the startsWith method on each string.

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const arr = ['ab', 'ac', 'ad'];

const result = arr.every(str => str.startsWith('a'));

console.log(result); // 👉️ true

On each iteration, we call the startsWith method on the string and return the result.