TypeError: toISOString is not a function in JavaScript

# TypeError: toISOString is not a function in JavaScript

The "TypeError: toISOString is not a function" error occurs when the toISOString() 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 toISOString() method on valid date objects.

Here is an example of how the error occurs.

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

// ⛔️ TypeError: toISOString is not a function
const result = d.toISOString();

The Date.now() function returns an integer, so calling the Date.toISOString() method on an integer caused the error.

# Only call the toISOString() method on valid Date objects

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

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// ✅ Works
const d1 = new Date().toISOString();
console.log(d1);

const d2 = new Date('Sept 24, 22 13:20:18').toISOString();
console.log(d2); // 👉️ 2022-09-24...

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

Note that if an invalid date is passed to the Date() constructor, you would get the "Invalid time value" RangeError.

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// ⛔️ RangeError: invalid time value
const d1 = new Date('invalid').toISOString();
console.log(d1);

You can console.log the value you are calling the toISOString method on to check if it's a valid Date object.

# Check if the value is a valid Date object

You could also conditionally check if the value is a Date object.

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const d1 = new Date();

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

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

We first check if the d1 variable stores a value with a type of object, because dates have a type of object.

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

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console.log(typeof null); // 👉️ "object"

Then we know we can safely call the toISOString() 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.