The `isNaN` function in JavaScript is used to determine whether a value is `NaN` (Not a Number). This function is particularly useful because `NaN` is a special numeric value that represents a result of an undefined or unrepresentable mathematical operation.
For example, in JavaScript, when you try to perform a mathematical operation that doesn’t result in a meaningful number, the result is `NaN`. Since `NaN` is not equal to any value, including itself, you cannot use the usual equality operators to check for `NaN`. This is where `isNaN` comes in handy.
Here’s how it works:
1. Argument: `isNaN` takes one argument, which is the value to be tested.
2. Return Value: It returns `true` if the argument is `NaN`; otherwise, it returns `false`.
For instance:
– `isNaN(NaN)` will return `true`.
– `isNaN(123)` will return `false`, since 123 is a number.
– `isNaN(“abc”)` will return `true`, because when “abc” is converted to a number, it results in `NaN`.
It’s important to note that `isNaN` will first try to convert the argument to a number if it is not already one, and then determines if the resulting value is `NaN`. This conversion can sometimes lead to unexpected results, especially with non-numeric values.
In modern JavaScript, there is also `Number.isNaN()` which is a more reliable way of checking for `NaN`, as it doesn’t force a conversion of the input and checks if the value is exactly `NaN`.
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