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Add reference Markdown: operators/variable-expression #59

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253 changes: 253 additions & 0 deletions reference/operators/variable-expression/$let.md
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---
title: $let
description: The $let operator allows defining variables for use in a specified expression, enabling complex calculations and reducing code repetition.
type: operators
category: variable-expression
---

# $let

The `$let` operator is used to define variables for use in a specified expression. It allows you to create temporary variables that can be referenced within the expression, making complex calculations more readable and preventing redundant computations.

## Syntax

```javascript
{
$let: {
vars: {
<var1>: <expression1>,
<var2>: <expression2>,
...
},
in: <expression>
}
}
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`vars`** | An object that defines the variables and their values. Each variable is assigned the result of an expression. |
| **`in`** | The expression that uses the variables defined in the `vars` object. Variables are referenced using `$$<variable_name>`. |

## Examples

Consider this sample document from the stores collection.

```json
{
"_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
"name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
"location": {
"lat": -89.2384,
"lon": -46.4012
},
"staff": {
"totalStaff": {
"fullTime": 8,
"partTime": 20
}
},
"sales": {
"totalSales": 75670,
"salesByCategory": [
{
"categoryName": "Wine Accessories",
"totalSales": 34440
},
{
"categoryName": "Bitters",
"totalSales": 39496
},
{
"categoryName": "Rum",
"totalSales": 1734
}
]
},
"promotionEvents": [
{
"eventName": "Unbeatable Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 7,
"Day": 2
}
},
"discounts": [
{
"categoryName": "Whiskey",
"discountPercentage": 7
},
{
"categoryName": "Bitters",
"discountPercentage": 15
},
{
"categoryName": "Brandy",
"discountPercentage": 8
},
{
"categoryName": "Sports Drinks",
"discountPercentage": 22
},
{
"categoryName": "Vodka",
"discountPercentage": 19
}
]
},
{
"eventName": "Steal of a Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 29
}
},
"discounts": [
{
"categoryName": "Organic Wine",
"discountPercentage": 19
},
{
"categoryName": "White Wine",
"discountPercentage": 20
},
{
"categoryName": "Sparkling Wine",
"discountPercentage": 19
},
{
"categoryName": "Whiskey",
"discountPercentage": 17
},
{
"categoryName": "Vodka",
"discountPercentage": 23
}
]
}
]
}
```

### Example 1: Basic variable usage

Calculate staff efficiency metrics using defined variables.

```javascript
db.stores.aggregate([
{ $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
{
$project: {
name: 1,
staffMetrics: {
$let: {
vars: {
fullTime: "$staff.totalStaff.fullTime",
partTime: "$staff.totalStaff.partTime",
totalSales: "$sales.totalSales"
},
in: {
totalStaff: { $add: ["$$fullTime", "$$partTime"] },
salesPerEmployee: { $divide: ["$$totalSales", { $add: ["$$fullTime", "$$partTime"] }] },
fullTimeRatio: { $divide: ["$$fullTime", { $add: ["$$fullTime", "$$partTime"] }] }
}
}
}
}
}
])
```

This query returns the following result.

```json
[
{
"_id": '40d6f4d7-50cd-4929-9a07-0a7a133c2e74',
"name": 'Proseware, Inc. | Home Entertainment Hub - East Linwoodbury',
"staffMetrics": {
"totalStaff": 39,
"salesPerEmployee": 3893.948717948718,
"fullTimeRatio": 0.5128205128205128
}
}
]
```

### Example 2: Nested $let expressions

Calculate location-based insights with nested variable definitions.

```javascript
db.stores.aggregate([
{ $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
{
$project: {
name: 1,
locationInsights: {
$let: {
vars: {
lat: "$location.lat",
lon: "$location.lon"
},
in: {
coordinates: { lat: "$$lat", lon: "$$lon" },
hemisphere: {
$let: {
vars: {
northSouth: { $cond: [{ $gte: ["$$lat", 0] }, "North", "South"] },
eastWest: { $cond: [{ $gte: ["$$lon", 0] }, "East", "West"] }
},
in: {
latitudeHemisphere: "$$northSouth",
longitudeHemisphere: "$$eastWest",
quadrant: { $concat: ["$$northSouth", " ", "$$eastWest"] }
}
}
},
distanceFromEquator: { $abs: "$$lat" },
distanceFromPrimeMeridian: { $abs: "$$lon" }
}
}
}
}
}
])
```

This query returns the following result.

```json
[
{
"_id": '40d6f4d7-50cd-4929-9a07-0a7a133c2e74',
"name": 'Proseware, Inc. | Home Entertainment Hub - East Linwoodbury',
"locationInsights": {
"coordinates": { "lat": 70.1272, "lon": 69.7296 },
"hemisphere": {
"latitudeHemisphere": 'North',
"longitudeHemisphere": 'East',
"quadrant": 'North East'
},
"distanceFromEquator": 70.1272,
"distanceFromPrimeMeridian": 69.7296
}
}
]
```