Function FromString

internal/jsnum/string.go:41–97

https://tc39.es/ecma262/2024/multipage/abstract-operations.html#sec-stringtonumber

(s string)

// https://tc39.es/ecma262/2024/multipage/abstract-operations.html#sec-stringtonumber
func FromString(s string) Number {
	// Implementing StringToNumber exactly as written in the spec involves
	// writing a parser, along with the conversion of the parsed AST into the
	// actual value.
	//
	// We've already implemented a number parser in the scanner, but we can't
	// import it here. We also do not have the conversion implemented since we
	// previously just wrote `+literal` and let the runtime handle it.
	//
	// The strategy below is to instead break the number apart and fix it up
	// such that Go's own parsing functionality can handle it. This won't be
	// the fastest method, but it saves us from writing the full parser and
	// conversion logic.

	s = strings.TrimFunc(s, isStrWhiteSpace)

	switch s {
	case "":
		return 0
	case "Infinity", "+Infinity":
		return Inf(1)
	case "-Infinity":
		return Inf(-1)
	}

	for _, r := range s {
		if !isNumberRune(r) {
			return NaN()
		}
	}

	if n, ok := tryParseInt(s); ok {
		return n
	}

	// Cut this off first so we can ensure -0 is returned as -0.
	s, negative := strings.CutPrefix(s, "-")

	if !negative {
		s, _ = strings.CutPrefix(s, "+")
	}

	if first, _ := utf8.DecodeRuneInString(s); !stringutil.IsDigit(first) && first != '.' {
		return NaN()
	}

	f := parseFloatString(s)
	if math.IsNaN(f) {
		return NaN()
	}

	sign := 1.0
	if negative {
		sign = -1.0
	}
	return Number(math.Copysign(f, sign))
}