---
schema_version: "1"
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title: leftJoin
collection: mulesoft
version: current
locale: en-us
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canonical_url: https://docs.mulesoft.com/dataweave/2.3/dw-arrays-functions-leftjoin
source_site: docs.mulesoft.com
content_type: documentation
content_hash: d3ea60683fd883c4683a1d0b2ffd52ac3b31c704755459843e16cc08be29578b
description: MuleSoft Documentation Site
release: "2.3"
latest: false
---
---
product: DataWeave
version: 2.3
---

# leftJoin

> For the full documentation index, see: https://docs.mulesoft.com/llms.txt

## leftJoin(Array<L>, Array<R>, (leftValue: L) -> String, (rightValue: R) -> String): Array<{ l: L, r?: R }>

Joins two arrays of objects by a given ID criteria.

`leftJoin` returns an array all the `left` items, merged by ID with any right items that meet the
joining criteria.

_Introduced in DataWeave 2.2.0. Supported by Mule 4.2 and later._

### Parameters

<table><colgroup><col> <col></colgroup><thead><tr><th>Name</th><th>Description</th></tr></thead><tbody><tr><td><p><code>left</code></p></td><td><p>The left-side array of objects.</p></td></tr><tr><td><p><code>right</code></p></td><td><p>The right-side array of objects.</p></td></tr><tr><td><p><code>leftCriteria</code></p></td><td><p>The criteria used to extract the ID for the left collection.</p></td></tr><tr><td><p><code>rightCriteria</code></p></td><td><p>The criteria used to extract the ID for the right collection.</p></td></tr></tbody></table>

### Example

This example shows how join behaves. Notice that it returns all objects from the left-side array
(`left`) but only joins items from the right-side array (`right`) if the values of the left-side
`user.id` and right-side `product.ownerId` match.

#### Source

```DataWeave
 %dw 2.0
 import * from dw::core::Arrays
 var users = [{id: "1", name:"Mariano"},{id: "2", name:"Leandro"},{id: "3", name:"Julian"},{id: "5", name:"Julian"}]
 var products = [{ownerId: "1", name:"DataWeave"},{ownerId: "1", name:"BAT"}, {ownerId: "3", name:"DataSense"}, {ownerId: "4", name:"SmartConnectors"}]
 output application/json
 ---
 leftJoin(users, products, (user) -> user.id, (product) -> product.ownerId)
```

#### Output

```json
[
  {
    "l": {
      "id": "1",
      "name": "Mariano"
    },
    "r": {
      "ownerId": "1",
      "name": "DataWeave"
    }
  },
  {
    "l": {
      "id": "1",
      "name": "Mariano"
    },
    "r": {
      "ownerId": "1",
      "name": "BAT"
    }
  },
  {
    "l": {
      "id": "2",
      "name": "Leandro"
    }
  },
  {
    "l": {
      "id": "3",
      "name": "Julian"
    },
    "r": {
      "ownerId": "3",
      "name": "DataSense"
    }
  },
  {
    "l": {
      "id": "5",
      "name": "Julian"
    }
  }
]
```