[khusamov]

Ext.define('User', {
            extend: 'Ext.data.Model',
            proxy: {
                type: 'memory',
                reader: {
                    type: 'json'
                }
            },
            fields: [
                {mapping: 'ErrorCode', name: 'name',  type: 'string'},
                {name: 'age',   type: 'int', convert: null},
                {name: 'phone', type: 'string'},
                {name: 'alive', type: 'boolean', defaultValue: true, convert: null}
            ]
        });

        var usr = Ext.create('User', {
            ErrorCode: 'test string'
        });

        console.log(
            // expected "test string"
            usr.get('name')
        )

песочница
https://fiddle.sencha.com/#view/editor&fiddle/283k