What is this syntax isNaN(+str!)

Question

Found it here

export function convertToUnit (str: string | number | null | undefined, unit = 'px'): string | undefined {
  if (str == null || str === '') {
    return undefined
  } else if (isNaN(+str!)) { // **** THIS LINE ****
    return String(str)
  } else {
    return `${Number(str)}${unit}`
  }
}

`+str` tries to convert a string (or anything, really) to a number and yields NaN if it could not convert. `isNaN` (is not a number, which checks for `NaN` type) checks if the conversion was successful. — pascalpuetz, Sep 15 '20 at 16:35

CertainPerformance · Accepted Answer · 2020-09-15T16:42:47.807

5

The expression +str! combines the (JavaScript operator) unary plus with the (TypeScript-exclusive operator) non-null assertion operator.

The ! asserts that str is not null nor undefined before the + is done. But, due to the str == null test done above, the str cannot be null nor undefined at that point: the assertion is unnecessary.

So, it's equivalent to

+str

which casts str to a number. This numeric value is then passed to isNaN, which will return true if the value is NaN, and will otherwise return false.

edited Sep 15 '20 at 16:42

answered Sep 15 '20 at 16:39

CertainPerformance

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It might also be worth noting that the `!` is a TypeScript feature. The usage of the `+` operator is standard JavaScript and not something specific to TypeScript. – Bart Hofland Sep 15 '20 at 16:41

score 2 · Answer 2 · answered Sep 15 '20 at 16:42

The +, or unary plus operator, converts an object (str in this case) to a number. It's similar to parseInt or parseFloat but not exactly the same. You can read more about it in this answer.

The ! asserts that str isn't null, to suppress any warnings that the compiler would otherwise give. You can read more here.

If str is not a number, then the + operation will return 'Not a Number', or NaN. isNaN will return true if NaN was returned, or false if str was a valid number.

What is this syntax isNaN(+str!)

2 Answers2

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