Initialize a JavaScript array with the specified values

Oftentimes, you will need to initialize an array with a given length and fill it with the same value. For example, you might want to create an array filled with 0 or null values. Other times, you might want to initialize an array with a repeated sequence of values. Luckily, these tasks are trivial in JavaScript.

Initialize an array with the specified value

The simplest and most common scenario is to initialize an array with the same value for all items. We can use the Array() constructor to create an array of the desired length and then use Array.prototype.fill() to fill it with the desired value. You can set up the default value to 0 if you don't supply one, for convenience.

const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);

initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]

Initialize an array with multiple values

A more complex scenario is to initialize an array with multiple values. Supplying an array of values is undesirable, as the value we want might be in itself the array. Instead, we will opt to use rest parameters, which will allow us to supply any number of arguments. This way, we can supply as many values as we want and even allow arrays to be treated as any other value.

Having decided on the interface, we can check the length of the values to handle each case differently. If there are no values, we'll still default to 0, and exactly one values will be treated the same as the previous code snippet.

Finally, if multiple values are supplied, we will use Array.from() to create an array of the desired length. Then, we will use the second argument of the method to map the array to the given values, using the modulo (%) operator to cycle through them.

const initializeArrayWithValues = (n, ...vals) => {
  if (vals.length === 0) return Array(n).fill(0);
  if (vals.length === 1) return Array(n).fill(vals[0]);
  return Array.from({ length: n }, (_, i) => vals[i % vals.length]);
};

initializeArrayWithValues(5); // [0, 0, 0, 0, 0]
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]
initializeArrayWithValues(5, 2, 3); // [2, 3, 2, 3, 2]
initializeArrayWithValues(3, [2, 3]); // [[2, 3], [2, 3], [2, 3]]