A lot of people that use JavaScript don't understand (or even want to understand) the apply and call functions that are built into every function.
In combination with being a bit confusing, a lot of times you can get around them by just adding an additional parameter or some extra code here and there, and so you just ignore them.
I just ran across a situation that shows a great use case for apply and hopefully will help someone understand how to use it.
The scenario is I have two arrays. One array of N points starting from 0 and one array of M points starting at an some positive integer X. When my JS code handles array M, I want to insert and overwrite positions X...M.length + X of array N. The brute force way would just be to loop through them all copying them as they go. I thought about it for a bit and gave myself a quick reminder of what functions exist for a JS array.
I thought about concat, but that only appends to the end. I thought about splice, but that only handles a list of parameters. Then I remembered that JS already has that covered with apply. Using apply will solve my issue of having an array instead of a list of parameters. The code for this is pretty simple. Given 2 arrays, and a position it will be this:
You might notice the [position, array2.length].concat code in there and wonder what that is. Well the first 2 parameters to splice are the position and the # of elements to move. The remaining parameters are the values to insert. So using concat to add the two together will make the apply call work.
Here is a live example:
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| Or apply me maybe? |
I just ran across a situation that shows a great use case for apply and hopefully will help someone understand how to use it.
The scenario is I have two arrays. One array of N points starting from 0 and one array of M points starting at an some positive integer X. When my JS code handles array M, I want to insert and overwrite positions X...M.length + X of array N. The brute force way would just be to loop through them all copying them as they go. I thought about it for a bit and gave myself a quick reminder of what functions exist for a JS array.
I thought about concat, but that only appends to the end. I thought about splice, but that only handles a list of parameters. Then I remembered that JS already has that covered with apply. Using apply will solve my issue of having an array instead of a list of parameters. The code for this is pretty simple. Given 2 arrays, and a position it will be this:
Array.prototype.splice.apply(array1,
[position, array2.length].concat(array2));
You might notice the [position, array2.length].concat code in there and wonder what that is. Well the first 2 parameters to splice are the position and the # of elements to move. The remaining parameters are the values to insert. So using concat to add the two together will make the apply call work.
Here is a live example:
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