Refer to Array in Declaration in Javascript

Question

I was wondering whether I could refer to the array I am declaring in Javascript in its own declaration... See code

var data = [["foo",23],["bar",data[0][1]+6]];

I would like that array to be ["foo",23]["bar",29]

Thank you!

• Icepickle: you cannot really use data in the array (you are creating a new local variable called data at that time. Why not create a variable that contains var fooArr = ["foo", 23], barArr = ["bar", foo[1]+6], data = [fooArr, barArr];. More interesting would be why you think that you need this kind of construct
• HMR: It's kind of a strange question or bad practical example. I wonder why you would do that since 23 is a hard coded number and 23+6 is 29 you may as well do var data = [["foo",23]["bar",29]] if the number 23 comes from a variable (let's say num) then you can do ["foo",num]["bar",num+6] if you have an existing array and need an array from this based on previous value you can use reduce or zip/map

Answer 1

The JavaScript engine would first parse the array initializer, it won’t find any definitions for "data" and will throw an error. So, it wouldn’t work.

• Sebastian Simon: To clarify: an error will be thrown, because data will be undefined at that point and no property 0 can exist on it. It won’t throw an error merely because data doesn’t exist (var data; is hoisted), but something like let or const will, because of that.

Answer 2

It won't work as your referring to a variable that was not created yet. At this point data it still undefined. So you need first finish the declaration statement and then refer to the variable.