Question

I was killing time reading the underscore.string functions, when I found this weird shorthand:

function count (str, substr) {
  var count = 0, index;
  for (var i = 0; i < str.length;) {
    index = str.indexOf(substr, i);
    index >= 0 && count++; //what is this line doing?
    i = i + (index >= 0 ? index : 0) + substr.length;
  }
  return count;
}

Legal: Think twice before using the function above without giving credit to underscore.string

---

I put the line alone here, so you don’t waste time finding it:

index >= 0 && count++;

I have never seen anything similar to that. I am clueless in what is doing.

Answer 1

index >= 0 && count++;

First part: index >= 0

returns true if index has a value that is greater than or equal to 0.

Second part: a && b

most C-style languages shortcut the boolean || and && operators.

For an || operation, you only need to know that the first operand is true and the entire operation will return true.

For an && operation, you only need to know that the first operand is false and the entire operation will return false.

Third Part: count++

count++ is equivalent to count += 1 is equivalent to count = count + 1

All together now

If the first operand (index >= 0) of the line evaluates as true, the second operand (count++) will evaluate, so it’s equivalent to:

if (index >= 0) {
  count = count + 1;
}

JavaScript nuances

JavaScript is different from other C-style languages in that it has the concept of truthy and falsey values. If a value evaluates to false, 0, NaN, "", null, or undefined, it is falsey; all other values are truthy.

|| and && operators in JavaScript don’t return boolean values, they return the last executed operand.

2 || 1 will return 2 because the first operand returned a truthy value, true or anything else will always return true, so no more of the operation needs to execute. Alternatively, null && 100 will return null because the first operand returned a falsey value.