Nightwatch.js get value from input and use it in next step

I'm trying to run Nightwatch script which will open a url, then it will getValue from an Input and will use that value in the next page.

Please see the following code:

var conf = require('../../nightwatch.conf.BASIC.js');

module.exports = {
    'nightwatch flow': function (browser) {
        var first_name;
        browser
            .url('http://example:3000')
            .getValue('input[name="first_name"]', function(result){
                first_name = result.value;
            })
            .setValue('input[name="amount"]', 101)
            .click('input[name=continue]')
            .clearValue('input[name="first_name"]')
            .setValue('input[name="first_name"]', first_name)
            .click('button[name=next]')
            .end();
    }
};

the setValue('input[name="first_name"]', first_name) gets "undefined"

first_name parameter is being updated inside a callback function. I need that setValue function will use the updated value. Thanks in advance