// First we illustrate copy by value.
var n = 1;        // Variable n holds the value 1.
var m = n;        // Copy by value: variable m holds a distinct value 1.

// Here's a function we'll use to illustrate pass-by-value.
// As we'll see, the function doesn't work the way we'd like it to.
function add_to_total(total, x)
    total = total + x;  // This line only changes the internal copy of total.

// Now call the function, passing the numbers contained in n and m by value.
// The value of n is copied, and that copied value is named total within the
// function. The function adds a copy of m to that copy of n. But adding
// something to a copy of n doesn't affect the original value of n outside
// of the function. So calling this function doesn't accomplish anything.
add_to_total(n, m);

// Now, we'll look at comparison by value. 
// In the line of code below, the literal 1 is clearly a distinct numeric
// value encoded in the program. We compare it to the value held in variable
// n. In comparison by value, the bytes of the two numbers are checked to
// see if they are the same.
if (n == 1) m = 2;   // n contains the same value as the literal 1; m is now 2.

Published in: JavaScript

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