Assume that we have the following short JavaScript code which runs on a browser that is ECMA 5 compatible:

<script>
    var x = [1, 2, 3, 4].map(function(x, y) { return x + y } );

    alert(x);
</script>

Will this code succeed or fail? and if it succeeds, what is the output of the alert?
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The code will work fine. The final result is [1, 3, 5, 7]. Let’s understand why we will have this result. The main idea of the previous expression is to understand how Array.prototype.map works.

According to ECMA documentation, Array.prototype.map creates a new array with the results of calling a provided function on every element in this array; this provided function represents the first parameter. This means that the following function will be called on every element in the array:

function(x, y) { return x + y } 

Array.prototype.map passes the array element and its index to the provided function; this means that the resulting array will be as follows:
[1 + 0, 2 + 1, 3 + 2, 4 + 3] ==> [1, 3, 5, 7].