An article to talk about NaN in JavaScript

In JavaScript, NaN is a special numeric value (the result of typeof NaN is number), which is not a number is an abbreviation, indicating that it is not a legal number.

1. Generation of NaN:

• A number that cannot be parsed

Number('abc') // NaN
Number(undefined) // NaN

• Failed operation

Math.log(-1) // NaN
Math.sqrt(-1) // NaN
Math.acos(2)  // NaN

• An operator is NaN

NaN + 1 // NaN
10 / NaN  // NaN

2. Notes

NaN is the only value that is not equal to itself:

NaN === NaN  // false

3. How To identify NaN

we can use the global function isNaN() to determine whether a value is a non-number (not used to determine whether Not the value NaN):

isNaN(NaN)  // true
isNaN(10)  // false

Why is isNaN() not used to determine whether it is the value NaN? Because isNaN doesn't work on non-numbers, the first thing it does is convert these values ​​to numbers, which may result in NaN, and then the function will incorrectly return true :

isNaN('abc')  // true

So we want to make sure that this value is NaN, we can use the following two methods:

• Method 1: Change isNaN() is combined with typeof to determine

function isValueNaN(value) {
	return typeof value === 'number' && isNaN(value)
}

• Method 2: Whether the value is not equal to itself (NaN is OnlyThe value with such characteristics)

function isValueNaN(value) {
	return value !== value
}

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