Solve - toUpperCase is not a function Error in JavaScript

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Borislav Hadzhiev

Last updated: Oct 20, 2021

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Solve - toUpperCase is not a function Error #

The "toUpperCase is not a function" error occurs when we call the toUpperCase() 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 toUpperCase method on strings.

typeerror touppercase is not a function

Here is an example of how the error occurs.

index.js
const str = {}; // ⛔️ TypeError: str.toUpperCase is not a function const result = str.toUpperCase();

We called the String.toUpperCase method on an object and got the error back.

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

index.js
const num = 123; const result = num.toString().toUpperCase();

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

index.js
const str = null; const result = typeof str === 'string' ? str.toUpperCase() : ''; console.log(result); // 👉️ ""

We used a 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.

If the value is a string, we return the result from calling the toUpperCase method on it, otherwise we return an empty string to be consistent.

If you want to convert all strings in an array to uppercase, use the map method to iterate over the array and call the toUpperCase method on each string.

index.js
const arr = ['hey', 'hi', 'hello']; const result = arr.map(str => str.toUpperCase()); // 👇️ ['HEY', 'HI', 'HELLO'] console.log(result);
The map method takes a function and calls the function on each element in the array. On each iteration, we call the toUpperCase method to convert the current string to uppercase.

If the error persists, console.log the value you're calling the toUpperCase method on and check it's type using the typeof operator.

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 toUpperCase() method.

Further Reading #

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