Is the following function legal and portable?
function(_, _, x){
return x;
}
Sometimes I want to write a callback that doesn't use the leftmost parameters so I wonder what is the most concise way to do so.
Conclusion:
function(_1, _2, x) is probably as short as it gets then.
It is valid in non-strict mode code, but invalid in strict mode code:
It is a SyntaxError if any Identifier value occurs more than once within a FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression.
Source: http://es5.github.com/#x13.1
Therefore, you may want to avoid this, since at one point in the future you will want to move on to strict mode...
You can use arguments, but yes that will work:
function test(_, _, x){
console.log(arguments);
return x;
}
console.log(test('a','b','c'));
Outputs:
["a", "b", "c"]
c
And here is what it prints if you try to use the _ argument:
["a", "b", "c"]
b
c
arguments. The whole point is ignoring them :)
var ignoreLeftParam = function(count, f) {
return function() {
f.apply(this, Array.prototype.slice.call(arguments, count));
}
}
ignoreLeftParam(2, function(x) {
return x;
});
Writing a general utility to ignore parameters might be considered neater.
_1, etc, but I am a sucker for functional programing so I love to see someone also though this.If you're running into problems with this kind of function signature, try rewriting it to accept a single options object with named properties. That way you can pass whatever combination of arguments you want.
E.g.:
function test(options) {
if('numberOfHands' in options)
console.log('I have ' + options.numberOfHands + ' hands!');
if('duration' in options)
console.log('I last for ' + options.duration);
}
Took me all of five seconds to test and apparently yes, this is legal.
I can see why you may want to do this. Take jQuery's $.get() method for example. Say your "success" callback function only had to use the third jqXHR argument.
$.get('url', function(_, _, jqXHR) {
// only use jqXHR
});
Though you may as well name the first two arguments x and y.
function(notUsed1,notUsed2,x)- or name the parameters for what they actually are and simply not use them. More readable, and you don't have to worry about it breaking in some obscure browser (or in strict mode of the popular browsers as per Šime's answer).