Undefined is not a function when using array.splice

Question

I am trying to remove some elements from an array, and use the splice functionality where it also returns those values. See the javascript below:

var blocksUsed = allNumbers.splice(0,3);

When trying to run this line (in conjunction with the entire function), I get an error saying the following:

Uncaught TypeError: undefined is not a function

I have spent quite some time looking for typographical errors in the function, but there don't seem to be any. I also ready that JQuery is the common troublemaker here, but I am not using JQuery.

The array is created by shuffling another array (the shuffle function isn't mine):

function shuffle(o){
    for(var j, x, i = o.length; i; j = Math.floor(Math.random() * i), x = o[--i], o[i] = o[j], o[j] = x);
    return o;
};

Which I then use in the following manner:

var allNumbers = shuffle(blocks);

I tried logging the allNumbers, which works correctly. The array gets shuffled correctly. Trying to do a splice on this however gives a same error.

Also note that I am doing the splicing inside a for-loop. Code below.

for (var i = 0; i < 4; i++){

    // Get a block of 3 blocks.
    var blocksUsed = allNumbers.splice(0,3); // This line gives the error.

    // Determine the color used for these blocks and remove it so that it doesn't get drawn again.
    var colorIndex = randomIntFromInterval(0,i);
    var colorUsed = otherColors[colorIndex];
    if(otherColors.indexOf(colorUsed) > -1){
        otherColors.splice(colorIndex, 1);
    }

    // For each block, set the color.
    for(var j = 0; j < 3; j++){
        blocksUsed[j].className = "block " + colorUsed;
    }
}

Why can I not use splice on this array? Note that I can provide the entire function, if desired. I have the entire function at this codepen for reference straight away.

Because of the comment:

allNumbers is a shuffled version of an array containing elements received in the following manner:

var blocks = document.getElementsByClassName('block');

Answer 1

var blocks = document.getElementsByClassName('block');

That is not an array. It is array like. You need to make it into an array.

var elems = document.getElementsByClassName('block');
var blocks = Array.prototype.slice.call( elems, 0 );

Answer 2

blocks is not an Array . It's a NodeList So you need to use [].slice.call which will take blocks, which is an array-like structure and return expected results.

var blocksArr = [].slice.call(blocks, 0); 
var blocksUSed = blocksArr.splice(0, 3);