JSFiddle - React, Tailwind, and code Playground

JavaScript

var myArray = ['2','1','1','1','1'];

indexPurge(myArray, 0,3);

function indexPurge(haystack,indexTarget, morphToValue){
	
    var toIntHaystack = [];
    
    for(var i = 0; i < haystack.length; i++){
        toIntHaystack.push(parseInt(haystack[i]));
    }
	
	console.log(toIntHaystack); //before
	
	var i = 0;
	
	var purgedValue = 0;

	do{
		console.log(i + ' - ' + toIntHaystack[i]);
		purgedValue += toIntHaystack[i];
		toIntHaystack.splice(i,1);
		if(purgedValue >= morphToValue){
			break;
		}
		i++;
	}while(i < toIntHaystack.length);
	
	
	
	toIntHaystack.splice(indexTarget,0,morphToValue); //after
	console.log(toIntHaystack);
	
}