JavaScript Reference
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Quick Reference

The JavaScript isNaN() method returns true if a value is NaN (not a number).

<!-- html element to place output -->
<p id="my_output"></p>
// variable
let my_number = isNaN('2022/12/31');

// output to the HTML element
document.getElementById('my_output').innerHTML = my_number;

Note

Don’t confuse this with the Number.isNaN() method where the value is NOT converted to a number and then tested as to whether it is not a number (NaN). The global .isNaN() method DOES convert the value to a number type first, and then tests whether it is not a number (NaN).

Output

true

Syntax

isNaN(value)

JavaScript Notes:

  • When using JavaScript, single or double quotation marks are acceptable and work identically to one another; choose whichever you prefer, and stay consistent
  • JavaScript is a case-sensitive language; firstName is NOT the same as firstname
  • Arrays count starting from zero NOT one; so item 1 is position [0], item 2 is position [1], and item 3 is position [2] … and so on
  • JavaScript variables must begin with a letter, $, or _
  • JavaScript variables are case sensitive (x is not the same as X)
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