JavaScript Array Undefined Items

This article discusses undefined items in JavaScript arrays, explaining the differences between explicitly undefined values and uninitialized array elements, and how array iteration behaves differently in these cases.

Jan 27, 2021 · 1 min read · 165 Words · -Views -Comments · Programming

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Example
Analysis
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🇨🇳

Example
Analysis
Related Discussions

Recently encountered an issue where array iteration counts were incorrect due to uninitialized elements. This led to the discovery that array iteration counts don’t necessarily match the array length. Here’s a summary of this pitfall.

Example

 let array = [undefined, , 1, 2, 3];
      console.log(array[0]); // undefined
      console.log(array[1]); // undefined

      array.forEach((item, idx) => {
        console.log(item, idx);
      });

Output result:

Analysis

Whether you assign undefined to an array item or leave it uninitialized, the value will be undefined
During iteration, assigned items will be traversed, but unassigned items are skipped directly
For example, when you need to control a fixed number of loops, you might write it like this. Note that fill() ensures array items are assigned as undefined. If you omit this, the iteration count will be 0
```
 new Array(10).fill().forEach((_, idx) => {
        console.log(idx + 1);
 });
```