Node Sequelize (MSSQL) - Login failed for user ''

More for older versions:
If you are using sequelize@4, then it seems there is a hidden requirement that you must use tedious@<=5.

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Which version of Sequelize are you using? If it's v5, According to Sequelize v5's document:

Sequelize now works with tedious >= 6.0.0

However, in its package.json, it does not depend on tedious at all. Since your program still runs, I guess you manually installed an older version of tedious before, which caused this strange problem.

Manually installing tedious of version>=6 should solve this problem, just like stated in its Getting started document page:

You'll also have to manually install the driver for your database of choice:
# One of the following: $ npm install --save pg pg-hstore # Postgres $ npm install --save mysql2 $ npm install --save mariadb $ npm install --save sqlite3 $ npm install --save tedious # Microsoft SQL Server