JavaScript “new Array(n)” and “Array.prototype.map” weirdness

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粉色の甜心
粉色の甜心 2020-11-22 02:40

I\'ve observed this in Firefox-3.5.7/Firebug-1.5.3 and Firefox-3.6.16/Firebug-1.6.2

When I fire up Firebug:

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  • 2020-11-22 03:39

    It appears that the first example

    x = new Array(3);
    

    Creates an array with undefined pointers.

    And the second creates an array with pointers to 3 undefined objects, in this case the pointers them self are NOT undefined, only the objects they point to.

    y = [undefined, undefined, undefined]
    // The following is not equivalent to the above, it's the same as new Array(3)
    y = [,,,];
    

    As map is run in the context of the objects in the array I believe the first map fails to run the function at all while the second manages to run.

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  • 2020-11-22 03:39

    I had a task that I only knew the length of the array and needed to transform the items. I wanted to do something like this:

    let arr = new Array(10).map((val,idx) => idx);
    

    To quickly create an array like this:

    [0,1,2,3,4,5,6,7,8,9]
    

    But it didn't work because: (see Jonathan Lonowski's answer a few answers above)

    The solution could be to fill up the array items with any value (even with undefined) using Array.prototype.fill()

    let arr = new Array(10).fill(undefined).map((val,idx) => idx);
    

    console.log(new Array(10).fill(undefined).map((val, idx) => idx));

    Update

    Another solution could be:

    let arr = Array.apply(null, Array(10)).map((val, idx) => idx);
    

    console.log(Array.apply(null, Array(10)).map((val, idx) => idx));

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