运营商的Javascript评估顺序

Question

Which of the following expressions will always precede left to right in all browsers(particularly IE6+, F3+, Opera 9+, Chrome)? For example the window should always alert first function then second function . In C they always suggest not to depend on the order of the evaluation of expressions. Is the same true for JavaScript or is Operator Precedence consistent?

function first(){
    alert('first function');
    return 0;
}
function second(){
    alert('second function');
    return 23;
}
first() + second();
first() - second();
first() * second();
first() / second();
first() < second();
first() > second();

Using mozilla it appears function evaluation should be consistent in all browsers, but obviously the standard isn't always followed.

Testing

After some test on browsershots.org it appears all browsers follow the standard.
Generally The exception is when relying on the the valueOf method in javascript. ValueOf definitely appears to be called backwards in specific cases for google chrome.

// The following alerts second then first in google chrome
first.valueOf = function(){alert('first');};
second.valueOf = function(){alert('second');};
first > second;

Answer 1

ECMAScript 5 specifies the order of evaluation of the operands for all operators. In the case of every operator in your code snip the evaluation order is left-to-right. I'm not sure anyone could answer about the behavior of all browsers though.

Edit: See also ECMAScript 3. Evaluation order is defined the same way.

Answer 2

Evaluating the expression into a value (eg involving a function call) is always done left to right.

However, once you are comparing two values, they are not converted into primitives in order to do the actual comparison in a left to right fashion. Try the following in Chrome, for example:

var l = {valueOf: function() { alert("L"); }};
var r = {valueOf: function() { alert("R"); }};

l < r; //alerts "L", then "R"
l > r; //alerts "R", then "L"

Answer 3

Operator precedence and order of evaluation are two entirely different things. In the expression "sqrt(9) + sqrt(16) * sqrt(25)" it is misleading to say that "the multiplication is done first.". It is correct to say "multiplication takes precedence over addition.".

The operations are:

1. sqrt(9)
2. sqrt(16)
3. sqrt(25)
4. 4 * 5
5. 3 + 20

The first three could be done in any order, or -- gasp -- simultaneously if you have a four-core CPU and a browser that can take advantage of it. 1 must be done before 5, 2 and 3 must be done before 4, and 4 must be done before 5. There are no other guarantees.

In the expression "a && (b / a)", JavaScript guarantees that a is evaluated first and b / a is not evaluated if a is zero. Many other languages including Fortran do not guarantee this, and can get a division-by-zero exception.

Answer 4

I'm not sure what your use-case is, but this might be an option:

function add() {
  var retval = 0;
  for (var i = 0; i < arguments.length; i++) {
    retval += arguments[i];
  }
  return retval;
}

function echoNum(num) {
  alert("Num: " + num);
  return num;
}

alert("Result: " + add(echoNum(1), echoNum(2)));