如何合并数组？及如何为JavaScript File对象添加preview属性？

Hey there! Let's break down your two questions step by step:

There are several practical ways to merge arrays in JavaScript, each suited to different scenarios:

• Using the concat() method
  This creates a brand new array without altering the original two arrays—perfect if you need to keep your source data intact.

  const arr1 = [1, 2, 3];
  const arr2 = [4, 5, 6];
  const mergedArray = arr1.concat(arr2);
  // Result: mergedArray = [1,2,3,4,5,6]; arr1 and arr2 stay unchanged
  

• Using the spread operator (...)
  A clean, modern approach that also returns a new array. You can merge multiple arrays at once with this method too.

  const arr1 = [1, 2, 3];
  const arr2 = [4, 5, 6];
  const mergedArray = [...arr1, ...arr2];
  // Result: mergedArray = [1,2,3,4,5,6]
  

• Using push() to modify the original array
  If you don't need to preserve the first array, this method adds all elements from the second array directly to the first one.

  const arr1 = [1, 2, 3];
  const arr2 = [4, 5, 6];
  arr1.push(...arr2);
  // Result: arr1 becomes [1,2,3,4,5,6]; arr2 remains the same
  

When you grab a File object via myItem.getAsFile(), it's a browser-built instance of the File type. You have two solid options to add your preview URL alongside its existing properties:

Option 1: Directly attach the preview to the File object #

JavaScript lets you add custom properties to built-in object instances, so this works seamlessly:

const item = myItem.getAsFile();
const preview = window.URL.createObjectURL(item);

// Add the preview property directly to the item
item.preview = preview;

// Now you can access item.preview, plus all original properties like item.name, item.size, etc.

Option 2: Create a new combined object (no modification to original File) #

If you want to leave the original File object untouched, create a new object that includes all its properties plus the preview URL. Since some File properties aren't enumerable, we'll explicitly include them:

const item = myItem.getAsFile();
const preview = window.URL.createObjectURL(item);

const itemWithPreview = {
  lastModified: item.lastModified,
  name: item.name,
  size: item.size,
  type: item.type,
  webkitRelativePath: item.webkitRelativePath,
  preview: preview,
  // Optional: include the original File if you need it later
  originalFile: item
};

// Use itemWithPreview.preview, itemWithPreview.name, etc.

Important reminder: When you're done using the preview URL (like when the image is no longer displayed), revoke it to free up browser memory:

window.URL.revokeObjectURL(preview);

内容的提问来源于stack exchange，提问作者Marian Rick