Make array numbers negative

You need to map the values to negative values.

function makeListNegative(array) {
  return array.map(x => x * -1);
}

var negativeList = makeListNegative([7, 2, 3, 4]);

console.log(negativeList);

function makeListNegative (array) {
    for (i = 0; i < array.length; i++); // 1
    return i * -1;                      // 2
}

1. Your for statement iterates (in the right range with the right interval), but with the semicolon at the end of the line, it perform no operation.

2. Then you return the negative value of i, which is the length of the array, not a value of the array or an array with all negative values.

If you like to get you traditional approach, you could push the value in every iteration to a new array and return the new array after the iteration.

function makeListNegative(array) {
    var i, l,
        temp = [];

    for (i = 0, l = array.length; i < l; i++) {
        temp.push(-array[i]);
    }

    return temp;
}

var negativeList = makeListNegative([7, 2, 3, 4]);

console.log(negativeList);

You could map the negative values with Array#map.

function makeListNegative(array) {
    return array.map(v => -v);
}

var negativeList = makeListNegative([7, 2, 3, 4]);

console.log(negativeList);

Your code

• returned every iteration
• had a semicolon in the wrong place
• made the index negative instead of the array item

It could have been fixed like this

function makeListNegative(array) {
  for (var i = 0; i < array.length; i++) { // make i local
    array[i] *= -1; // negate the array item
  }
  return array; // return the manipulated array
}

var negativeList = makeListNegative([7, 2, 3, 4]);
console.log(negativeList);

Alternatively use Array.map - here in standard JS - the fat arrow => is ES6+ and does not work in IE

var negativeList = [7, 2, 3, 4].map(function(num) { return -num })
console.log(negativeList);

You can impress your senior devs by using ES6 syntax:

const makeListNegative = array => array.map(num => -num)

const negativeList = makeListNegative([7, 2, 3, 4]);
console.log(negativeList);

And if you don't want to create a function:

const negativeList = [7, 2, 3, 4].map(num => -num);
console.log(negativeList);

If you are using Babel or Typescript in your project, you don't need to worry about IE support because your code will be transpiled to ES5 and will be supported on all browsers.

This will not work as you are returning from your for loop. Try using

Array.map()

You can use map to return negative values

var negativeList=  [7, 2, 3, 4].map(function(value){
  return -v;
})

Since everyone solved the problem using map, here is how to solve using [forEach][1],

var makeNeg = function(arr){
  arr.forEach(function(value, index){
    arr[index] = arr[index]*-1;
  });
  return arr;
}

console.log(makeNeg([7, 2, 3, 4]))