Why Mongoose doesn’t validate on update?

As of Mongoose 4.0 you can run validators on update() and findOneAndUpdate() using the new flag runValidators: true.

Mongoose 4.0 introduces an option to run validators on update() and
findOneAndUpdate() calls. Turning this option on will run validators
for all fields that your update() call tries to $set or $unset.

For example, given OP’s Schema:

const ClientSchema = new Schema({
  name: {type: String, required: true, trim: true}
});

const Client = mongoose.model('Client', ClientSchema);

Passing the flag on each update

You can use the new flag like this:

const id = req.params.id;
const client = req.body;
Client.update({_id: id}, client, { runValidators: true }, function(err) {
  ....
});

Using the flag on a pre hook

If you don’t want to set the flag every time you update something, you can set a pre hook for findOneAndUpdate():

// Pre hook for `findOneAndUpdate`
ClientSchema.pre('findOneAndUpdate', function(next) {
  this.options.runValidators = true;
  next();
});

Then you can update() using the validators without passing the runValidators flag every time.