I've been trying to get data from an API, a async call is used to parse the data. I've be naively iterating over the responses with forEach, calling await in the forEach function, and wondering why it is not producing the result I expected, i.e. wait for the data to settle.

NG:

async function getData() {
  output = ""
  results.forEach(async (block) => {   
    var b = await abc.parse(block)
    output += b
  })
}

Solution

Use a standard for or for..of to loop over the list:

async function getData() {
  output = ""
  for (const block of results) {
    var b = await abc.parse(block)
    output += b
  }
}

Suppose you have an array of object:

var array = [   {a:13, b:false },
        {a:7, b:true },
        {a:78, b:true } ]       

The old, ECMAScript 3 way of checking for objects, in which b is true would be a simple for-loop:

var results = [];
for (var i=0; i
Ok, that's working, but not very elegant nor concise. Luckily, ECMAScript 5 offers a nicer way:
var results = array.filter( function(item) {
    return item.b;      // if item.b is true
});

    

Ok, looping over an array is something that we do all the time, for most of us it has become a second nature.

var myArray = ["a", "b", "c", "z"];

for (int i=0; i
But I recently came across a Javascript loop that puzzled me slightly.
var myArray = ["a", "b", "c", "z"];

for (var i=0,f ; f=myArray[ i++]; ) {
    console.log(f);
}

Let's have a closer look what's happening here. As a reminder, For-statements have the following structure:
for (initiation; condition; increment) { block } 
In the initiation, variables are initiated; Condition tests this var, and if it returns true, continues another loop; and here's something funny; the increment is left blank. Don't panic, because we are already incrementing i at the condition test.
This is equivalent:
var myArray = ["a", "b", "c", "z"];

for (var i=0,f ; f=myArray[ i]; i++) {
    console.log(f);
}

This, however, is not the same:
var myArray = ["a", "b", "c", "z"];

for (var i=0,f ; f=myArray[ ++i]; ) {
    console.log(f);
}

i gets incremented before it is used to traverse myArray, therefor resulting in this output:
b
c
z

In nice and simple arrays like in this example, it should not make any difference to test the array length at every time coming around, but a larger ones it might be beneficial.
As usual, take care that the array does not contain any falsy objects. And don't loop an object that way.
Thanks to the Re-introduction to Javascript https://developer.mozilla.org/en/a_re-introduction_to_javascript  for clarifying that issue for me.
Update
Turns out things are not so clear after all. Let's consider the following case: I have an array of numbers, I'd like to get the sum of all numbers in the array.
One way to do it:
var myArray = [1,2,3,5,7];
for (var i=0, sum=0; i
So, shouldn't this work too...?
var myArray = [1,2,3,5,7];
for (var i=0, sum=0; sum = myArray[i]; i++) {};
console.log(sum);       // undefined

Apparently not... I am trying to find out why not.
This, on the other hand works:
for (var i=0, sum=0; this[i]; sum+=this[i++]){};