Question

I am trying to generate a 2D n*m array in javascript full of zeros. What is the fastest way of doing that?

I know the simple for loop would be enough to set all the elements to 0, but what I would like to know, why can’t I do that with mapping. For example with the underscore lib (or even the native map)

_.map(Array(n),function(a){return 0}) // makes {undefined,undefined,...}

while

_.map([1,2,3,5,6],function(a){return 0}) // makes {0,0,0,0,0}

Can anyone explain if I can fill an empty array with a map function and how, or why not?

PS: There is a trivial solution to my problem, I am just asking this cause I would like to learn more, and I cant find a good enough answer on google. Thank you

Answer 1

Typical JavaScript .map() functions ignore array members that are undefined. That’s why Array(n) doesn’t work.

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You could easily add a method to Array.prototype to do a quick fill…

Array.prototype.fill = function(n,v) {
    n = n || this.length;
    for( var i = 0; i < n; i++ ) 
        this[i] = v === void 0 ? i : v;
    return this;
};

then…

Array().fill(3); // [0,1,2]

Array().fill(3, 0); // [0,0,0]