Does Javascript pass array to function by reference or value?

user_1829237 0
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As my understand, Javascript pass object by reference, and a array also a object but when I create a array of integer then passing it to a function as below code:

function testFunc(outTestArray) {
  var aiTemp = [1,2,3,4];

  /*Using slice(0) to clone array */
  outTestArray = aiTemp.slice(0);
}

var aiTest = Array.apply(null, Array(4)).map(Number.prototype.valueOf, 0);
testFunc(aiTest);

console.log(aiTest.toString()); // aiTest still [0,0,0,0]

I also know that the slice(0) function just return a shallow copy of array, but in case the array is only a array of integer. So my question is why the data of aiTest is not modified?

javascript-objects arrays javascript
3 Answers

Function arguments are like new variable assignments, it's not like references or pointers in C. It's more something like this:

function testFunc(...args) {
  var outTestArray = args[0];
  var aiTemp = [1,2,3,4];

  /*Using slice(0) to clone array */
  outTestArray = aiTemp.slice(0);
}
...

As you can see, in the code above you are cloning the array and assigning it the variable outTestArray scoped within the function, which is not accessible outside of it.

You could use a closure to achieve what you want:

var outerArray;
function testFunc(array) {
  var aiTemp = [1,2,3,4];
  outerArray= aiTemp.slice(0);
}

or even better, just return a new array:

function getArray() {
  return [1,2,3,4];
}

var aiTest = getArray();
...
user_19741320

In your code, testFunc gets a reference to the original array which is copied into argument outTestArray. In the body of the function you are making outTestArray point to a different array. If you want to change the original array, then you could say

outTestArray.splice(0, outTestArray.length, ...aiTemp)

Note that spread operator is being used for aiTemp.

user_91161510
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