How to initialize an array's length in JavaScript?

Question

Most of the tutorials that I've read on arrays in JavaScript (including w3schools and devguru) suggest that you can initialize an array with a certain length by passing an integer to the Array constructor using the var test = new Array(4); syntax.

After using this syntax liberally in my js files, I ran one of the files through jsLint, and it freaked out:

Error: Problem at line 1 character 22: Expected ')' and instead saw '4'.
var test = new Array(4);
Problem at line 1 character 23: Expected ';' and instead saw ')'.
var test = new Array(4);
Problem at line 1 character 23: Expected an identifier and instead saw ')'.

After reading through jsLint's explanation of its behavior, it looks like jsLint doesn't really like the new Array() syntax, and instead prefers [] when declaring arrays.

So I have a couple questions:

First, why? Am I running any risk by using the new Array() syntax instead? Are there browser incompatibilities that I should be aware of?

And second, if I switch to the square bracket syntax, is there any way to declare an array and set its length all on one line, or do I have to do something like this:

var test = []; test.length = 4; 

Answer 1

• Array(5) gives you an array with length 5 but no values, hence you can't iterate over it.

• Array.apply(null, Array(5)).map(function () {}) gives you an array with length 5 and undefined as values, now it can be iterated over.

• Array.apply(null, Array(5)).map(function (x, i) { return i; }) gives you an array with length 5 and values 0,1,2,3,4.

• Array(5).forEach(alert) does nothing, Array.apply(null, Array(5)).forEach(alert) gives you 5 alerts

• ES6 gives us Array.from so now you can also use Array.from(Array(5)).forEach(alert)

• If you want to initialize with a certain value, these are good to knows...
  Array.from('abcde'), Array.from('x'.repeat(5))
  or Array.from({length: 5}, (v, i) => i) // gives [0, 1, 2, 3, 4]