In the last article we talked about some of the unary and binary operators available in Javascript, namely the arithmetic operators, the comparison operators and the increment/decrement operators. In this article we continue our discussion of the Javascript operators; we will discuss how computers represent data, and bitwise operators.

The Power of Javascript: Operators continued - Assignment Operators Example (Page 5 of 5 )

The following code uses all the combined assignment operators in one script.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <title>Hello World</title> <script language="JavaScript" type="text/javascript"> // x represented in binary as 00000101 var x = 3;

var result = 6; result += x; document.write(result + "<br>"); // we need to assign result the value 6 after using the assignment operator // and just before using it again. var result = 6; result -= x; document.write(result + "<br>"); var result = 6; result *= x; document.write(result + "<br>"); var result = 6; result /= x; document.write(result + "<br>"); var result = 6; result %= x; document.write(result + "<br>");

// 6 = 00000110 // 3 = 00000011

var result = 6; result &= x; document.write(result + "<br>"); var result = 6; result |= x; document.write(result + "<br>"); var result = 6; result ^= x; document.write(result + "<br>"); var result = 6; result <<= x; document.write(result + "<br>"); var result = 6; result >>= x; document.write(result + "<br>"); </script> </head> <body> </body> </html>

When you save the code into a file, then load it into your browser, you will get the following results:

After the variable declaration statements we add the value of x to the result. Note that I say "add" not "assign," because when you assign a value to a variable it replaces the old value which will be destroyed. So, using the statement result += x; the value of x will be added to the value of result; now result contains the value 9.

The next assignment statement produces the value 3 because result -= x; means that result is subtracted from x, then the produced value is assigned to result. As you can see we have assigned the value 6 to result before this assignment statement, and before all the assignment statements in this example, because using operators such as +=, -=, *= or &= modify (assign) the value of result. We need to set result to 6 before using it, so we know that x = 3 and result = 6, and test whether we are following what's going on or not. The statement result *= x; multiplies result by x, then assigns the value to result. The statement result /= x divides result on x, then assigns the value to result.

The statement result &= x; may confuse you, so let's analyze what it means. It means result = result & x; which instructs the interpreter to do the bitwise AND operation on result. Then x assigns the value to result, which equals 6. This is represented in binary as 00000110. x is represented in binary as 00000011, so result & x produces:

00000110 & 00000011 --------- 00000010

00000010 equals two in decimal, which is assigned to the variable result. The statement result |= x; assigns the value of result | x to result, so we will set the resultant value's bits to 1 if any of the positioned matched bit is 1. Let's do it:

00000110 | 00000011 --------- 00000111

00000111 equals 7 in decimal, which is assigned to the variable result and written to the page.

The statement result ^= x; assigns the value of result ^ x to result. The XOR bitwise operator sets the resultant's value bits to 1 if only one of the positioned matched bit is 1. If both of them are 1 it sets the resultant bit to 0. Let's do it:

00000110 ^ 00000011 --------- 00000101

00000101 equals 5 in decimal, which is assigned to the variable result and written to the page.

The statement result <<= x; left shifts the value of result by the value of x, then assigns the value to result. It's like result = 6 << 3; 6 in binary is represented as 00000110, and when we shift this value 3 bits to the left the value becomes 00110000, which equals 48. This is the value that is assigned to result and written to the Web page. I think that now you understand how these operators work.

On the other hand, the statement result >>= x; assigns result the value of result >> x, which means that we need to right shift the value of the variable result by the value of x on the binary representation. Six equals 00000110, so when we shift it three times to the right the binary value will be 00000000, which means 0 in decimal. I hope that now you understand the idea of bitwise operators, although you will not use it in the range of client-side Javascript programming. It's good for building a programming bckground.

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