TypeError: next is not a function in pre-save hook (Mongoose v6+)

I was following this backend series in 2025 and encountered an issue in the User model related to Mongoose version updates.

The Error:

TypeError: next is not a function

Reason:

In Mongoose v6+, when using async middleware, Mongoose does not pass the next argument automatically. So this code from the tutorial:

userSchema.pre("save", async function (next) {
    if (!this.isModified("password")) return next();
    this.password = await bcrypt.hash(this.password, 10);
    next();
});

Leads to the error because next is undefined.

Fix for Mongoose v6+:

Remove the next argument and the next() calls:

userSchema.pre("save", async function () {
    if (!this.isModified("password")) return;
    this.password = await bcrypt.hash(this.password, 10);
});