The update command supports let parameters, but the db.collection.update() shell helper does not.

The following update command works:

db.runCommand({
    update: db.cakeSales.getName(),
    updates: [
        {q: {$expr: {$eq: [“$salesTotal”, “$$targetTotal”], u: [{$set: {salesTotal: “$$newTotal”}}]}
    ],
    let : {targetTotal: 2150, newTotal: 2155}
});

But the corresponding command using the shell helper fails with errmsg: “Use of undefined variable: targetTotal”.

db.cakeSales.update(
   {$expr: {$eq: [“$salesTotal”, “$$targetTotal”],
   {$set: {salesTotal: “$$newTotal”,
   {let : {targetTotal: 4350, newTotal: 4200}}
);