Javascript: Write a function that takes in a number, and returns an array with that number in it that many times

Question

I'm a complete newbie to programming and mathematical concepts (took Math 101 in college) so I'm struggling with this problem:

Write a function that takes in a number, and returns an array with that number in it that many times.

Here's the code I've got so far:

    function numReturn(x) {
         var newArray = [];
         if (typeof x === "number") {
             return newArray.push[x]* x;
         } else {
             return null;
         }
     }

Here's my thought process:

1. Create a function that can take in a number, x.
2. Within that function, create a blank array so you can push values to it later.
3. Check if the typeof value entered in for x is a number. If it is, return it pushed to the end of the blank array. Otherwise, return null.

When I put this in the Javascript console and plug a value in, it comes back undefined. Anyone have some pointers for me?

Answer 1

function a(i) {
    var a = new Array(i);
    return a.fill(i);
}

or return new Array(i).fill(i);, for short. Test:

a(4)
// --> [4, 4, 4, 4]

---

Array.prototype.fill() is an ES6 method and is not yet universally implemented. Chrome and Firefox have it, IE does not - but there is a polyfill available.

Compare: http://kangax.github.io/compat-table/es6/#test-Array.prototype_methods_Array.prototype.fill

Answer 2

In order to do something an arbitrary number of times, one uses a loop. There are several loops, but here the for loop is most appropriate.

A for loop has the following structure:

for(var i = 0; i < x; i++) {
    // ^initializer
    //         ^condition
    //                ^increment
    //body
}

The initializer is the first thing that is done when entering the loop. IN this case, it means a variable named i is set to 0. Then the condition x is checked. If the condition i < x holds, the loop is executed: the body is executed. After the body is executed, the increment is performed (here i++), and then the condition is rechecked, if the condition still holds, the loop is executed again, and so on.

You can apply this concept as follows:

function numReturn(x) {
    var newArray = [];
    if (typeof x === "number") {
        for(var i = 0; i < x; i++) {
            newArray.push(x);
        }
        return newArray;
    } else {
        return null;
    }
}

Answer 3

This: newArray.push[x]* x does not push x times. The * operator just multiplies numbers, always and only. You want to push x times, so use a for like this:

for (var i = 0; i < x; i++ )
      newArray.push(x);

and then return newArray.

Answer 4

Taking from answer of Most efficient way to create a zero filled JavaScript array?

function numReturn(x){
    return Array.apply(null, Array(x)).map(Number.prototype.valueOf,x);
}
console.log(numReturn(10)); // [10, 10, 10, 10, 10, 10, 10, 10, 10, 10]

Answer 5

var n = 7
var result = Array.apply(null, {length: n}).map(function(){return n})

// Demo output
document.write(JSON.stringify(result))

As function:

function numReturn(x) {
     if (typeof x === "number") {
         return Array.apply(null, {length: n}).map(function(){return n})
     } else {
         return null;
     }
 }