If you need the File use uri.fsPath
If you need the Workspace Folder use uri.path
if(vscode.workspace.workspaceFolders !== undefined) {
let wf = vscode.workspace.workspaceFolders[0].uri.path ;
let f = vscode.workspace.workspaceFolders[0].uri.fsPath ;
message = `YOUR-EXTENSION: folder: ${wf} - ${f}` ;
vscode.window.showInformationMessage(message);
}
else {
message = "YOUR-EXTENSION: Working folder not found, open a folder an try again" ;
vscode.window.showErrorMessage(message);
}
More detail can get from VS Code API