How to display current values of VS Code’s predefined variables (such as “${workspaceFolder}” or “${fileWorkspaceFolder}”)?

Old answer:

There may be a better way but you could run a

//  "preLaunchTask": "Echo vars" in your debug launch like:

{
    "name": "Chrome : Launch with sourcemaps",
    "type": "chrome",
    "request": "launch",
    "url": "http://localhost:3000",
    "webRoot": "${workspaceRoot}",
    "sourceMaps": true,
    "runtimeArgs": [
    "--remote-debugging-port=9222"
    ],
    "preLaunchTask": "Echo vars"
},

in your launch task and then in tasks.json add:

{
   "label": "Echo vars",
   "command": "echo",
   "args": [
     "${env:USERNAME}",
     "workspaceFolder = ${workspaceFolder}"
   ],
   "type": "shell"
},

Those values will be echoed to the terminal.

EDIT:

Because a later version of vscode now supports sending variables to the terminal this simpler keybinding will print out values in the terminal:

[
    {
        "key":  "alt+q",
        "command": "workbench.action.terminal.sendSequence",
        "args": {
            // "text": "echo ${env:USERNAME}",  // this works
            "text" : "echo file="${file}" : workspaceFolder="${workspaceFolder}"\u000D"
        }
    }
]

then Alt–q prints out the values.

The \u000D at the end is just a return.