CODEWITHSUNDEEP
javascriptarraysarray-splice
Frei Zhang
Frei Zhang

Reputation: 417

javascript array splice without index

I was wondering about this piece of code:

var numbers, _ref;
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

[].splice.apply(numbers, [3, 4].concat(_ref = [-3, -4, -5, -6])), _ref;
alert(numbers);

From here, the result is [0, 1, 2, -3, -4, -5, -6, 7, 8, 9] and can anyone explain me about this?

Upvotes: 0

Views: 2234

Answers (3)

Matt Sach
Matt Sach

Reputation: 1170

function.apply(context, argsArray) calls function in the given context, passing argsArray as the arguments for function.

In this case, function is [].splice, which takes the following parameters, in this this order:

  1. index - at which to start changing the array
  2. howMany - elements to remove, starting at index
  3. element1,...,elementN - elements to insert into the array at index

[3,4].concat(_ref = [-3, -4, -5, -6]) evaluates to an array by concatenating the two arrays together, giving [3, 4, -3, -4, -5, -6]. This is the argsArray passed to .apply(), so that:

  1. index == 3 (start at index 3)
  2. howMany == 4 (remove 4 elements)
  3. element1,...,elementN == -3, -4, -5, -6 (the elements to insert at index 3, after removal)

Thus .apply() is causing the .splice() function to run in the context of the numbers array, removing elements at indices 3, 4, 5 and 6, and then inserting the elements -3, -4, -5 and -6 between "2" and "7" in the original array.

Edit: See RobG's answer for a summary of what the original code is equivalent to (rather than an explanation of its parts).

Upvotes: 1

RobG
RobG

Reputation: 147403

Your code resolves to the following variable declarations:

var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
var _ref = [-3, -4, -5, -6];

And these expressions: 

numbers.splice(3, 4, -3, -4, -5, -6);
_ref;
alert(numbers);

Upvotes: 1

Prusse
Prusse

Reputation: 4315

[3, 4].concat(_ref = [-3, -4, -5, -6]) evals to [3, 4, -3, -4, -5, -6] and [].splice.apply(numbers, [3, 4, -3, -4, -5, -6])) to numbers.splice(3, 4, -3, -4, -5, -6) causing 4 elements stating from index 3 to be removed and the elements "-3, -4, -5, -6" to be inserted at the index 3. See splice.

Upvotes: 0

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