Knex updating with integers?

SQL query that I need to convert into Knex:

UPDATE wallabies SET age = age + 1 WHERE id = 4;

What I have:

knex('giraffes').where('id', 4).update({age: `${age}+1`})

How do I set it to age + 1 without using a string?

parseInt(value) try that or do Number(value) boxing

Is the age property already a number or is it a string?