# parse method

double parse (String source, [ @deprecated double onError(String source) ])

Parse `source` as an double literal and return its value.

Accepts an optional sign (`+` or `-`) followed by either the characters "Infinity", the characters "NaN" or a floating-point representation. A floating-point representation is composed of a mantissa and an optional exponent part. The mantissa is either a decimal point (`.`) followed by a sequence of (decimal) digits, or a sequence of digits optionally followed by a decimal point and optionally more digits. The (optional) exponent part consists of the character "e" or "E", an optional sign, and one or more digits.

Leading and trailing whitespace is ignored.

If the `source` is not a valid double literal, the `onError` is called with the `source` as argument, and its return value is used instead. If no `onError` is provided, a FormatException is thrown instead.

The `onError` function is only invoked if `source` is a String with an invalid format. It is not invoked if the `source` is invalid for some other reason, for example by being `null`.

Examples of accepted strings:

    "3.14"
    "  3.14 \xA0"
    "0."
    ".0"
    "-1.e3"
    "1234E+7"
    "+.12e-9"
    "-NaN"

The `onError` parameter is deprecated and will be removed. Instead of `double.parse(string, (string) { ... })`, you should use `double.tryParse(string) ?? (...)`.

## Implementation

    external static double parse(String source,
        [@deprecated double onError(String source)]);