How to push data into a "JSON" data type column in Postgresql

For anyone who happens to land on this question, the solution using Knex.js is:

knex('table')
      .where('id', id)
      .update({
        arrayColumn: knex.raw(`arrayColumn || ?::jsonb`, JSON.stringify(arrayToAppend))
})

This will produce a query like:

update tableName 
  set weightsovertime = arrayColumn || $1::json
where id = 20;

Where $1 will be replaced by the value of JSON.stringfy(arrayToAppend). Note that this conversion is obligatory because of a limitation of the Postegre drive

array_append is for native arrays - a JSON array inside a jsonb column is something different.

Assuming your weightsovertime is a jsonb (or json) you have to use the concatenation operator : ||

e.g:

update the_table
  set weitghtsovertime = weightsovertime||'[{"weight": 55, "date": "1996-03-02"}]'
where id = 20;

Online example: http://rextester.com/XBA24609