$ composer init -n
$ composer config extra.array '[]'
$ composer config extra.boolean false
$ composer config extra.null null
$ composer config extra.number 42
$ composer config extra.object '{}'
$ composer config extra.string 'Hello, world!'
$ cat composer.json
{
    "require": {},
    "extra": {
        "array": "[]",
        "boolean": "false",
        "null": "null",
        "number": "42",
        "object": "{}",
        "string": "Hello, world!"
    }
}

{
    "require": {},
    "extra": {
        "array": [],
        "boolean": false,
        "null": null,
        "number": 42,
        "object": {},
        "string": "Hello, world!"
    }
}

$ git config -h
usage: git config [<options>]

...

Type
    -t, --type <>         value is given this type
    --bool                value is "true" or "false"
    --int                 value is decimal number
    --bool-or-int         value is --bool or --int
    --path                value is a path (file or directory name)
    --expiry-date         value is an expiry date

...