In JavaScript, how do I mutate the value of an array inside of a function? I know that happens when using certain array methods, but it doesn't seem to work for normal assignment.
var arr = [4]
function changeArray(arr) {
arr = [1,2,3];
}
// arr is still equal to [4]. I want it to equal [1,2,3].
It's my understanding that this code doesn't change the value of "arr" outside of the function. How can I do it in a way that mutates the array that was passed as an argument?
You can use .splice:
arr.splice(0, arr.length, 1, 2, 3);
Or you can empty the array and .push the values:
arr.length = 0;
arr.push(1, 2, 3);
// or given an array of values
arr.push.apply(arr, newValues);
However, make sure it is clear that the function changes the array in place. While there are native functions that does this (like Array#sort), I'd argue that this is not as common as simply returning a new array.
It's my understanding that this code doesn't change the value of "arr" outside of the function.
Correct. Assigning a new value to a variable or property never changes the value of another variable or property (exceptions: global scope and with statements).
You basically tried to change the value of a variable, not mutate the value itself.
JavaScript is call/pass/assign by value (Wikipedia also mentions "by sharing"), not by reference.
.push + .apply: arr.push.apply(arr, newElements);