Question 246243 in Javascript Question posted in
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Javascript – Missing comma in JS array literal – Why does this compile?

I forgot a comma initializing a 2D array in javascript and, to my surprise, it compiled. E.g.:

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

console.log(amIMissingSomething) // [undefined, [7, 8, 9]]

I expected a syntax error to occur. However, after a whole lot of time, I learned this is valid javascript and evaluates to: [undefined, [7, 8, 9]]. What’s going on here?

Tags:

2

Answers


  1. 0

    It is getting parsed as:

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

    the comma operator returns the last expression so basically:

    amIMissingSomething = [
    [1,2,3][6],
    [7,8,9]
]

    and since the first array doesn’t have 7 elements it evaluates to undefined

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  2. 0

    It’s interpreted as array[index]

    See property accessor

    const a = [1, 2, 3][0]

console.log(a)

    The result of expession 4, 5, 6 is 6, see comma operator

    const arrayChangeHandler = {
  get: function(target, property) {
    console.log('getting ' + property + ' for ' + target);
    return target[property];
  },
};

const originalArray = [1, 2, 3];
const proxyToArray = new Proxy( originalArray, arrayChangeHandler );

proxyToArray[4, 5, 6]
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