Edit: This question is out of date as the Polyfill example has been updated. I'm leaving the question here just for reference. Read the correct answer for useful information on bitwise shift operators.
Question:
On line 7 in the Polyfill example of the Mozilla Array.prototype.indexOf page they comment this:
var length = this.length >>> 0; // Hack to convert object.length to a UInt32
But the bitwise shift specification on Mozilla clearly states that the operator returns a value of the same type as the left operand:
Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operand.
So shouldn't length receive the standard 64-bit float value? Or can someone point out to me where the hack starts?
The toString() method in Javascript is used with a number and converts the number to a string. It is used to return a string representing the specified Number object. The toString() method is used with a number num as shown in the above syntax using the '. ' operator.
In JavaScript parseInt() function (or a method) is used to convert the passed in string parameter or value to an integer value itself. This function returns an integer of base which is specified in second argument of parseInt() function.
The ECMAScript specification states that the value is converted to UInt32 in step 5 and 8 of http://www.ecma-international.org/ecma-262/5.1/#sec-11.7:
11.7.3 The Unsigned Right Shift Operator ( >>> )
Performs a zero-filling bitwise right shift operation on the left operand by the amount > specified by the right operand.
The production
ShiftExpression : ShiftExpression >>> AdditiveExpression
is evaluated as follows:
- Let
lref
be the result of evaluatingShiftExpression
.- Let
lval
beGetValue(lref)
.- Let
rref
be the result of evaluatingAdditiveExpression
.- Let
rval
beGetValue(rref)
.- Let
lnum
beToUint32(lval)
.- Let
rnum
beToUint32(rval)
.- Let
shiftCount
be the result of masking out all but the least significant 5 bits ofrnum
, that is, computernum & 0x1F
.- Return the result of performing a zero-filling right shift of
lnum
byshiftCount
bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.
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