How can I hide the menu bar in visual studio code?

If you want to toggle the menu bar with Alt, see this setting:

// Control the visibility of the menu bar. A setting of 'toggle' means that the menu bar is hidden and a single press of the Alt key will show it. By default, the menu bar will be visible, unless the window is full screen.

 "window.menuBarVisibility": "toggle",

The default is "default", change it to "toggle" in your user settings..

You can hide the menu bar by clicking the Toggle Menu Bar option in the View menu.

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To bring the menu bar back, as pointed out by pramesh-bajracharya in the comments:

Ctrl + Shift + P will open a command window and you can type Toggle Menu Bar to toggle it on and off.