Solve - toISOString is not a function Error in JavaScript

Solve - toISOString is not a function Error #

The "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();

We called the Date.now() function, which returns an integer and tried to call the Date.toISOString() method on it, which caused the error.

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 and see if it's a valid Date object.

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 && 'toISOString' in d1) {
  const result = d1.toISOString();
  console.log(result);
}

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

Then we check if the variable is not equal to null. Unfortunately, if you check the type of null - 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 toISOString method on the object.