这里我重新创建了遗留的Math.random函数来支持in between range，然后修复了range[0，arr_length-1]，这应该会对您的情况起到很好的作用。
Clarification: He's also never going to get other index but 0 because the usual数学随机returns a value in [0-1]! That's why I had to recreate the数学随机method for him and then just fix the new range to [0-arr_length-1]. And that guarantees him to get each time a random index in the correct range.
此外，您还需要调用您的函数以获得结果！

(function(){
  const originalMathRandom = Math.random;
  Math.random = function(min=0, max=1) {
    return originalMathRandom() * (max - min) + min;
  }
}());

var names = ["Hameed", "David", "Maleek", "Ola"];

function whosPaying(names){
    var output;
    var arr_length = names.length;
    var index = Math.floor(Math.random(0, arr_length-1));
    output = names[index] + " is going to pay for lunch today!";
     
    return output;
}

console.log( whosPaying(names) );

这个管用！

var names = ["Hameed", "David", "Maleek", "Ola"];

function whosPaying(names){

    var arr_length = names.length;
    var posInArray = Math.floor(Math.random() * arr_length );
    var randomPerson = names[posInArray];
    var output = randomPerson + " is going to buy lunch today!";

    return output
    
}

whosPaying(names);

函数whosPaying（名称）{

var Numberofpeople= names.length;
var randompersonposition = Math.floor(Math.random() * Numberofpeople);
var randomperson = names[randompersonposition];

return randomperson + " is going to buy lunch today!";

}