TypeError: date.getDate is not a function in JavaScript

# TypeError: date.getDate is not a function in JavaScript

The "TypeError: date.getDate is not a function" occurs when the getDate() method is called on a value that is not a date object.

To solve the error, convert the value to a date before calling the method or make sure to only call the getDate() method on valid date objects.

Here is an example of how the error occurs.

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const date = Date.now();
console.log(date); // 👉️ 1639...

// ⛔️ TypeError: date.getDate is not a function
const result = date.getDate();

The Date.now() function returns an integer, so we cannot call the Date.getDate() method on it.

# Only call the getDate() method on valid date objects

To solve the error, make sure to only call the getDate() method on valid date objects.

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const d1 = new Date().getDate();
console.log(d1); // 👉️ 16

const d2 = new Date('Sept 24, 22 13:20:18').getDate();
console.log(d2); // 👉️ 24

You can get a date object by passing a valid date to the Date() constructor.

If an invalid date is passed to the Date() constructor, you would get NaN (not a number) value back.

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const d1 = new Date('invalid').getDate();
console.log(d1); // 👉️ NaN

You can console.log the value you are calling the getDate method on and see if it's a valid Date object.

# Check if the value is a Date object before calling getDate()

You could conditionally check if the value is a Date object in the following way.

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const d1 = new Date();

if (typeof d1 === 'object' && d1 !== null && 'getDate' in d1) {
  const result = d1.getDate();
  console.log(result); // 👉️ 16
}

Our if condition uses the logical AND (&&) operator, so for the if block to run, all of the conditions have to be met.

Then we check if the variable is not equal to null. Unfortunately, if you check the type of null with console.log(typeof null), you will get an "object" value back, so we have to make sure the value is not null.

Then we know we can safely call the getDate() method on the object.

This approach is called duck-typing.

When using duck-typing, we simply check if the object implements specific properties or methods and if it does, we assume it's an object of the correct type.