why is an empty array == 0 in js code example

Example 1: javascript empty array

arr = [];   // set array=[]

//function
const empty = arr => arr.length = 0;
//example
var arr= [1,2,3,4,5];
empty(arr) // arr=[]

Example 2: javascript empty array

var colors = ["red","blue","green"];
    colors = []; //empty the array