Example of guard clause

user.service.js

/*
 * Here important code is written inside long if block
 */
function login(username, password) {
    const user = await UserSchema.find({ email: username, password: password })

    if (user) {
        // Important code...
    } else {
        return new Error('Invalid credentials')
    }
}

/*
 * Here user is asserted at the beginning and the error has been returned thus stopping the further unnecessary execution
 */
function login(username, password) {
    const user = await UserSchema.find({ email: username, password: password })

    /*
     * Guard Clause
     */
    if (!user) {
        return new Error('Invalid credentials')
    }

    // Important code...
}