How does sequelize.sync() work, specifically the force option?

The OP was asking what force: false does, which is what I wanted to know too, so here's the rest.

The major takeaway, for me, was that the individual fields aren't synced (which is what I was hoping for, coming from the Waterline ORM). Meaning, if you have force: false and the table exists, any field additions/modifications/deletions you have won't be executed.

• beforeSync hooks are run
• table is dropped if force: true
• table is created with if not exists
• indexes are added if necessary
• afterSync hooks are run

Here's the current code from the github repo for reference:

lib.model.js

Model.prototype.sync = function(options) {
  options = options || {};
  options.hooks = options.hooks === undefined ? true : !!options.hooks;
  options = Utils._.extend({}, this.options, options);

  var self = this
    , attributes = this.tableAttributes;

  return Promise.try(function () {
    if (options.hooks) {
      return self.runHooks('beforeSync', options);
    }
  }).then(function () {
    if (options.force) {
      return self.drop(options);
    }
  }).then(function () {
    return self.QueryInterface.createTable(self.getTableName(options), attributes, options, self);
  }).then(function () {
    return self.QueryInterface.showIndex(self.getTableName(options), options);
  }).then(function (indexes) {
    // Assign an auto-generated name to indexes which are not named by the user
    self.options.indexes = self.QueryInterface.nameIndexes(self.options.indexes, self.tableName);

    indexes = _.filter(self.options.indexes, function (item1) {
      return !_.some(indexes, function (item2) {
        return item1.name === item2.name;
      });
    });

    return Promise.map(indexes, function (index) {
      return self.QueryInterface.addIndex(self.getTableName(options), _.assign({logging: options.logging, benchmark: options.benchmark}, index), self.tableName);
    });
  }).then(function () {
    if (options.hooks) {
      return self.runHooks('afterSync', options);
    }
  }).return(this);
};

(More or less) formal docs and API reference can be found at https://sequelize.org/docs/v6/core-concepts/model-basics/#model-synchronization

To your question: force: true adds a DROP TABLE IF EXISTS before trying to create the table - if you force, existing tables will be overwritten.