# 实现 map

```javascript
 Array.prototype.map = function (fn, context = this) {
      if (typeof fn !== 'function') {
        throw new TypeError(fn + 'is not a function')  
      }
      const arr = this
      const result = new Array(ary.length);
      
      for (let i = 0; i < arr.length; i++) {
        // fix稀疏数组的情况
        if (i in arr) {
            result[i] = fn.call(context, arr[i], i, arr);
        }
      }
      return result
  }
```

使用 reduce 来实现 map

```javascript
Array.prototype.map = function(fn, context = this){
    if (typeof fn !== 'function') {
      throw new TypeError(fn + 'is not a function'); 
    }
    return this.reduce((acc, value, index) => {
      // fix稀疏数组的情况
      if (index in list) {
        acc[index] = fn.call(context, value, index, this);
      }
      return acc;
    }, []); // 注意这里的[]不能省
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://mm.ricky.moe/javascript/chang-jian-han-shu/shi-xian-map.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.