Proposal

Add a toInteger property be to the Number constructor, for converting values to IEEE-754 double precision integers, exactly as ECMA-262’s ToInteger internal method. It could be implemented like this:

(function (global) {
    var abs = Math.abs,
        floor = Math.floor,
        isFinite = global.isFinite,
        isNaN = global.isNaN;
        
    function sign(n) { return (n < 0) ? -1 : 1; }
 
    Object.defineProperty(Number, 'toInteger', {
        value: function toInteger(value) {
            var n = +value;
            if (isNaN(n))
                return +0;
            if (n === 0 || !isFinite(n))
                return n;
            return sign(n) * floor(abs(n));
        },
        configurable: true,
        enumerable: false,
        writable: true
    });
})(this);

There is currently no efficient or convenient way in the language to determine if a number is an exact integer.

— Douglas Crockford 2010/12/15 20:27 — Brendan Eich 2011/04/28 00:45

The blurb at the bottom seems to be about Number.isInteger, not toInteger.

It is already possible to invoke ECMA-262’s ToInteger function. Evaluating x | 0 produces ToInteger(x).

Contrary to the blurb at the top, the implementation of Number.toInteger given here is not the same as ECMA-262’s ToInteger. ToInteger(Infinity) is 0, but Number.toInteger(Infinity) returns Infinity. ToInteger(0×100000001) is 1, because ToInteger truncates the result to 32 bits; but Number.toInteger(0×100000001) is 0×100000001.

— Jason Orendorff 2011/12/09 18:55