CODEWITHSUNDEEP
javascriptfor-loop
geckob
geckob

Reputation: 8130

What does this for loop means?

I found this snippet in this minified JS by Google: 

_ga.utils.getValidColumnGroups = function(a) {
    for (var b = [], c = 0, d; d = _ga.metadata.items[c]; ++c) 
      d = d.attributes.group, 
      a && (d = _ga.utils.getNormalizedName(d)), 
      -1 == b.indexOf(d) && b.push(d);

    return b
};

Without focusing too much specifically on what this function is doing,

Upvotes: 1

Views: 68

Answers (3)

AxelH
AxelH

Reputation: 14572

I wonder how this loops will end when the conditions statement is assigning values and not the typical comparison operator?

d = _ga.metadata.items[c]

The condition can be write without any operator, like a simple infinite loop

while(true){}

this can be write like 

var infinite = true;
while(infinite){}

but also

var infinite;
while(infinite = true){} //this is a frequente mistake witch and up with infinite loop instead of while( infinite== true);

Basicly, you first set the value then check the condition.

In you example, this work for every other variant that a condition read as false like an undefined

What will happen especially with variable d in this line?

var b = [], c = 0, d;

This is the declaration part of the loop. You are allowed to declare as many as you want in the first part of a for(;;) loop.

Upvotes: 0

Domino
Domino

Reputation: 6768

As in most languages, the for loop is described as follows:

for(a; b; c) d;
  • Run instruction a
  • Run instruction b and convert the result to boolean.
  • If the boolean was true, run instructions d and c and evaluate instruction b again, else end the loop.

Since in JavaScript collections return undefined when you try to access beyond their length, and undefined converts to the boolean false, you can iterate through an array-like object like this:

for(var i=0, item; item = myArray[i]; ++i) {
  // code here
}

Since you're probably going to use the item anyway, it's a good idea to store it in a variable to make the code more efficient and clean. It's even more useful with objects that aren't exactly arrays, such as a NodeList, because their length property actually takes some time to compute (this can vary between browsers). The Google JavaScript Style Guide actually recommends doing this for that reason.

-1 == b.indexOf(d) && b.push(d);

This is a very, very ugly code practice. It abuses the && operator to do this:

if(b.indexOf(d) == -1) b.push(d);

It saves 1 character and makes the code a lot less readable. Never do that.

Upvotes: 0

RobG
RobG

Reputation: 147403

I wonder how this loops will end when the conditions statement is assigning values and not the typical comparison operator?

d = _ga.metadata.items[c]

The result of that expression is the value assigned to d, so the loop will end if d is assigned a falsey value (i.e. if _ga.metadata.items[c] returns a falsey value like undefined, 0, null, etc.).

What will happen especially with variable d in this line?

var b = [], c = 0, d;

That is simply some chained variable declarations. It declares b, c and d and initialises them with an empty array, the number zero and undefined (the default when variables are declared) respectively.

Upvotes: 1

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