@loopback/docs
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Documentation for LoopBack 4
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Markdown
lang: en
title: 'API docs: repository.buildlookupmap'
keywords: LoopBack 4.0, LoopBack 4
sidebar: lb4_sidebar
editurl: https://github.com/strongloop/loopback-next/tree/master/packages/repository
permalink: /doc/en/lb4/apidocs.repository.buildlookupmap.html
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[Home](./index.md) > [@loopback/repository](./repository.md) > [buildLookupMap](./repository.buildlookupmap.md)
## buildLookupMap() function
Returns a map which maps key values(ids) to instances. The instances can be grouped by different strategies.
<b>Signature:</b>
```typescript
export declare function buildLookupMap<Key, InType, OutType = InType>(list: InType[], keyName: StringKeyOf<InType>, reducer: (accumulator: OutType | undefined, current: InType) => OutType): Map<Key, OutType>;
```
## Parameters
| Parameter | Type | Description |
| --- | --- | --- |
| list | <code>InType[]</code> | an array of instances |
| keyName | <code>StringKeyOf<InType></code> | key name of the source |
| reducer | <code>(accumulator: OutType | undefined, current: InType) => OutType</code> | a strategy to reduce inputs to single item or array |
<b>Returns:</b>
`Map<Key, OutType>`