This test checks the behavior of the Array.prototype.fill() On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS [0, 0, 0, 0, 0].fill() is [undefined, undefined, undefined, undefined, undefined] PASS [0, 0, 0, 0, 0].fill(3) is [3, 3, 3, 3, 3] PASS [0, 0, 0, 0, 0].fill(3, 1) is [0, 3, 3, 3, 3] PASS [0, 0, 0, 0, 0].fill(3, 1, 3) is [0, 3, 3, 0, 0] PASS [0, 0, 0, 0, 0].fill(3, 1, 1000) is [0, 3, 3, 3, 3] PASS [0, 0, 0, 0, 0].fill(3, -2, 1000) is [0, 0, 0, 3, 3] PASS [0, 0, 0, 0, 0].fill(3, -2, 4) is [0, 0, 0, 3, 0] PASS [0, 0, 0, 0, 0].fill(3, -2, -1) is [0, 0, 0, 3, 0] PASS [0, 0, 0, 0, 0].fill(3, -2, -3) is [0, 0, 0, 0, 0] PASS [0, 0, 0, 0, 0].fill(3, undefined, 4) is [3, 3, 3, 3, 0] PASS [ , , , , 0].fill(3, 1, 3) is [, 3, 3, , 0] PASS successfullyParsed is true TEST COMPLETE