The method on arrays is used to take two arrays and concatenate them into one. It takes as many arrays as you like - so you can concatenate many arrays at once: concat Array.concat(value1, value2, value3, ...valueN) Each item added to the method must be of type array. Let's look at a simple example. Below, we combine two arrays: concat let myArray = [ 1, 2, 3, 4, 5 ]; let newArray = [ 'n', 'a', 'e', 'k' ] let mergedArray = myArray.concat(newArray); console.log(mergedArray); // [ 1, 2, 3, 4, 5, 'n', 'a', 'e', 'k' ] does not remove duplicates - but if you are interested in a data structure that does, . concat you might want to read my guide on Javascript sets The method creates a shallow copy of the array it's applied to, with the addition of any other arrays added to the end. concat That means that although it seems like a new array is created, it still has a connection with the original array you modified. In fact, every array you mention in is a shallow copy of the original. Consider the following example: concat let myArray = [ { name: "John" }, 1, 2, 3, 4, 5 ]; let newArray = [ { name: "Jacob" }, 'a', 'e', 'k' ] let mergedArray = myArray.concat(newArray); myArray[0].name = "Jingleheimer"; newArray[0].name = "Schmidt"; newArray[1] = 5; console.log(mergedArray, myArray); // [ { name: "Jingleheimer" }, 1, 2, 3, 4, 5, { name: "JSchmidtacob" }, 'a', 'e', 'k' ] You might expect that changing on and does not affect - but it does. However, has no effect on . name myArray newArray mergedArray newArray[1] = 5 mergedArray The basic premise is that the arrays we used in maintain the same reference in memory as the original arrays. Updating updates both the and the original arrays since they are all stored in the same place. However, typing tells Javascript to put a completely new value in position of `mergedArray. concat name mergedArray newArray[1] [1] This is important to remember when you run into weird Javascript array errors. ## Concating multiple arrays. We've looked at how you can concatenate two arrays, but concatenating more works in much the same way. You list all your arrays in the function like so: concat let myArray = [ 1, 2, 3, 4, 5 ]; let newArray = [ 'n', 'a', 'e', 'k' ] let anotherArray = [ 7, 1, 5, 7 ] let mergedArray = myArray.concat(newArray, anotherArray); console.log(mergedArray); // [ 1, 2, 3, 4, 5, 'n', 'a', 'e', 'k', 7, 1, 5, 7 ] The arrays are merged in the order they are mentioned. ## Concatenating nested arrays. Nested arrays are concatenated in the same way. The outer array is removed, meaning that the nested elements become part of the first array: let myArray = [ [1, [2, 3], 4], 5 ]; let newArray = [ ['n', 'a'], 'e', 'k' ] let mergedArray = myArray.concat(newArray); console.log(mergedArray); // [ [1, [2, 3], 4], 57, ['n', 'a'], 'e', 'k' ] ## Values in concatenated arrays If you do pass a simple value to , it will treat it as an array element. That means if you passed in rather than , it would coerce it to anyway: concat 'value' ['value'] ['value'] let myArray = [ 1, 2, 3, 4, 5 ]; let mergedArray = myArray.concat(1, 2, 3); console.log(mergedArray); // [ 1, 2, 3, 4, 5, 1, 2, 3 ] Also published here.
