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# lodash._bindcallback
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lodash._bindcallback is a specialized callback binding utility that provides optimized function binding with `this` context and argument count specifications. It's part of the Lodash utility library ecosystem, exported as a standalone Node.js module for efficient callback management.
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## Package Information
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- **Package Name**: lodash._bindcallback
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- **Package Type**: npm
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- **Language**: JavaScript
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- **Installation**: `npm install lodash._bindcallback`
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## Core Imports
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```javascript
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var bindCallback = require('lodash._bindcallback');
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```
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## Basic Usage
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```javascript
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var bindCallback = require('lodash._bindcallback');
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// Basic function binding with context
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function greet(name) {
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  return this.prefix + ' ' + name;
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}
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var context = { prefix: 'Hello' };
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var boundGreet = bindCallback(greet, context);
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console.log(boundGreet('World')); // "Hello World"
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// Optimized binding with argument count
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function processItem(value, index, collection) {
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  return this.transform(value) + index;
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}
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var processor = { transform: function(v) { return v.toUpperCase(); } };
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var boundProcessor = bindCallback(processItem, processor, 3);
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console.log(boundProcessor('hello', 0, ['hello'])); // "HELLO0"
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```
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## Capabilities
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### Callback Binding
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Creates a bound version of a function with specified `this` context and optional argument count optimization.
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```javascript { .api }
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/**
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 * A specialized version of baseCallback which only supports this binding
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 * and specifying the number of arguments to provide to func.
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 * 
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 * @param {Function} func The function to bind.
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 * @param {*} thisArg The this binding of func.
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 * @param {number} [argCount] The number of arguments to provide to func.
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 * @returns {Function} Returns the callback.
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 */
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function bindCallback(func, thisArg, argCount);
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```
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**Parameters:**
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- `func` (Function): The function to bind. If not a function, returns identity function.
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- `thisArg` (*): The `this` binding context for the function. If undefined, returns original function.
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- `argCount` (number, optional): Number of arguments to optimize for. Supports optimized implementations for 1, 3, 4, and 5 arguments.
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**Returns:**
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- (Function): A bound callback function optimized for the specified argument count.
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**Behavior:**
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- Returns identity function if `func` is not a function
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- Returns original function if `thisArg` is undefined  
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- Provides optimized implementations for argument counts 1, 3, 4, and 5
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- Falls back to general `apply` pattern for other argument counts
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**Usage Examples:**
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```javascript
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var bindCallback = require('lodash._bindcallback');
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// Example 1: Basic binding without argument count
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function multiply(a, b) {
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  return this.factor * a * b;
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}
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var math = { factor: 2 };
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var boundMultiply = bindCallback(multiply, math);
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console.log(boundMultiply(3, 4)); // 24
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// Example 2: Optimized for single argument (argCount: 1)
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function transform(value) {
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  return this.prefix + value + this.suffix;
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}
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var formatter = { prefix: '[', suffix: ']' };
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var boundTransform = bindCallback(transform, formatter, 1);
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console.log(boundTransform('test')); // "[test]"
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// Example 3: Optimized for collection iteration (argCount: 3)
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function processElement(value, index, collection) {
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  return this.base + value + index;
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}
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var processor = { base: 'item-' };
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var boundProcess = bindCallback(processElement, processor, 3);
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// Optimized for array map/forEach operations
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// Example 4: Optimized for reduce operations (argCount: 4)  
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function accumulate(acc, value, index, collection) {
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  return acc + this.weight * value;
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}
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var reducer = { weight: 0.5 };
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var boundAccumulate = bindCallback(accumulate, reducer, 4);
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// Optimized for array reduce operations
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// Example 5: Edge case - non-function input
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var notAFunction = "not a function";
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var result = bindCallback(notAFunction, {}); // Returns identity function
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// Example 6: Edge case - undefined thisArg
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var originalFunc = function() { return 'test'; };
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var unchanged = bindCallback(originalFunc, undefined); // Returns originalFunc
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```
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## Performance Optimizations
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The function provides specialized implementations for common argument patterns:
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- **1 argument**: Optimized for simple transformations and filters
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- **3 arguments**: Optimized for array iteration methods (map, forEach) with `(value, index, collection)`
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- **4 arguments**: Optimized for reduce operations with `(accumulator, value, index, collection)`
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- **5 arguments**: Optimized for comparison operations with `(value, other, key, object, source)`
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For other argument counts, it falls back to the general `Function.prototype.apply` approach.
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## Error Handling
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- **Invalid function input**: Returns identity function instead of throwing
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- **Undefined context**: Returns original function unchanged
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- **Missing arguments**: Handles gracefully with default behavior
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## Implementation Notes
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This is an internal Lodash utility exposed as a standalone module. It's designed for:
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- High-performance callback binding scenarios
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- Functional programming patterns requiring context binding
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- Integration with other Lodash utilities and methods
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- Zero-dependency lightweight function binding
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The module is part of Lodash 3.x's modular architecture, allowing selective importing of specific utilities without the full Lodash library.