MongoDB $ifNull

In MongoDB, $ifNull is an aggregation pipeline operator that allows you to specify a value to use in place of null.

The way that it works is, you provide an expression and a replacement expression. If the expression evaluates to a non-null value, that expression is returned. But if the expression evaluates to a null value, $ifNull returns the value of the replacement expression.

A null value includes instances of undefined values or missing fields.

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