LANGUAGE » JAVASCRIPT

Array

Last updated:
Reference:developer.mozilla.org

Basic ​

js
let arr = [2, 3, 5];
arr.length;
arr = Array(size).fill(value);
Array.isArray(myvar)
let empty = Array.isArray(myvar) && myvar.length

Generating an array from an array-like object:

js
arr = Array.from(arrayLikeObject);

Methods ​

MethodDescription
arr.indexOf(element)Returns the element's index, or -1 if not present.
arr.includes(element)Returns true if the element exists, false otherwise.
arr.join(',')Concatenates all elements using the separator and returns a string.
arr.push(element1, ...)Append elements to an array.
arr.pop()Removes the last element and return it.
arr.shift()Removes the first element and return it.
arr.slice()Returns a shallow copy of a portion of an array. [start; end] (end not included)
arr.splice(startIndex, deleteCount, item1, ...)Changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.
arr1.concat(arr2)Creates a new array with arr1 and arr2 elements merged.

Example:

js
arr.slice(2, 5)  // Same as arr[2:5] in python

Remove by value:

js
const index = arr.indexOf(value);
if (index !== -1) {
  arr.splice(index, 1);
}

With callback ​

js
arr.map(n => n * 2)
arr.reduce((accumulator, current) => accumulator + current, initialValue)
arr.forEach((currentValue, index, array) => doSomething())
arr.find(element => element > 4)  // first found element
arr.filter(element => element > 4)  // array that satisfies the condition

To iterate an array and run it async, use:

js
// Sequence
for (const file of files) {
  const contents = await fs.readFile(file, 'utf8');
  console.log(contents);
}

// Parallel
await Promise.all(files.map(async (file) => {
  const contents = await fs.readFile(file, 'utf8')
  console.log(contents)
}));