JSFiddle - React, Tailwind, and code Playground

JavaScript

var numbers = [
    [10, 20],
    [19, 40],
    [40, 60],
    [70, 80],
];

var merge = function(arr) {
    // copy and sort the array
    var result = arr.slice().sort(function(a, b) {
            return a[0] > b[0];
        }),
        i = 0;
    
    while(i < result.length - 1) {
        var current = result[i],
            next = result[i+1];
        
        // check if there is an overlapping
        if(current[1] >= next[0]) {
            current[1] = Math.max(current[1], next[1]);
            // remove next
            result.splice(i+1, 1);
        } else {
            // move to next
            i++;
        }
    }
    return result;
};

var mergedNumbers = merge(numbers);

console.log('numbers', numbers);
console.log('mergedNumbers', mergedNumbers);