Delete all elements in an array - Function appends 'undefined' at the array end

Question

Here is my code

var x = [];

function random(min,max) {
  return Math.floor(Math.random() * (min-max))+min;
}
function random2(a, b) {
  for (let i = 0; i < a; i++) {
    x.push(random(0,b));
  }
}
random2(5, 100);
console.log(x); // [ -43, -27, -38, -21, -79 ]

x.splice(0, x.length);
x.push(random2(5,100));
console.log(x); // [ -24, -97, -99, -43, -66, undefined ]

I simply wanna remove all the elements in the array then add new elements in it. But when I try to do it with the code above, undefined is also adding to the array.
How can I prevent it?

Possible duplicate of [How do I empty an array in JavaScript?](https://stackoverflow.com/questions/1232040/how-do-i-empty-an-array-in-javascript) — kemicofa ghost, Nov 16 '18 at 13:21
You're asking a wrong question. You should ask "why `undefined` is appearing to the array" instead of what you assume causing your real problem. — Teemu, Nov 16 '18 at 13:23

Nina Scholz · Accepted Answer · 2018-11-16T13:30:01.840

You need not to puish the function call, which returns undefined, but just call the function random2, because the function itselft add the elements to the array.

function random(min, max) {
    return Math.floor(Math.random() * (min - max)) + min;
}

function random2(a, b) {
    for (let i = 0; i < a; i++) {
        x.push(random(0, b));
    }
}

var x = [];

random2(5, 100);
console.log(x);

x.length = 0;          // better performance than x.splice(0, x.length)
random2(5,100);        // call without using push
console.log(x);        // no undefined anymore

A better approach is to return an array in random2, because this function does not access an outer defined array. To push the values, you could take the spread syntax.

function random(min, max) {
    return Math.floor(Math.random() * (min - max)) + min;
}

function random2(a, b) {
    return Array.from({ length: a }, _ => random(0, b));
}

var x = random2(5, 100);
console.log(x);

x.length = 0;          
x.push(...random2(5, 100));
console.log(x);

@nina, Out of the context, isn't it should be `random(a,b)` inside `random2` method, assuming the OP need to create `min` number of random numbers between `min` and `max` values ? Even the OP is wrong in his implementation. — Vignesh Raja, Nov 16 '18 at 13:40

score 1 · Answer 2 · answered Nov 16 '18 at 13:28

1

To empty an array, there are multiple ways as explained here with some benchmark results and explanation regarding their performance.

As an aggregation, asssume var a = [1,2,3,4,5]

a = []
a.length = 0
a.splice(0, a.length)
a = new Array()
while(a.pop()){}
while(a.shift()){}

You have called the function random2 inside the push method. So random2 method first inserts the values in the array x and returns the default value undefined (Reference), which in turn gets pushed into the array. Hence the value.

answered Nov 16 '18 at 13:28

Vignesh Raja

7,927
1
33
42

Thank you. That's correct, couldn't realise that first but thank to Nina, helped me realise that. – uhbc Nov 16 '18 at 13:33
1

Glad you got it. :) – Vignesh Raja Nov 16 '18 at 13:34
@uhbc Out of the context, isn't it should be `random(a,b)` inside `random2` method, assuming you need to create `min` number of random numbers between `min` and `max` values ? – Vignesh Raja Nov 16 '18 at 13:44

score 0 · Answer 3 · answered Nov 16 '18 at 13:17

0

Set the length to zero

x.length = 0;

answered Nov 16 '18 at 13:17

Josef Fazekas

447
5
11

Delete all elements in an array - Function appends 'undefined' at the array end

3 Answers3