I have nothing but approval and joy for negative offsets in substr and slice. I think it’s pretty intuitive and much easier to read than the equivalent with messing around with subtracting the string’s length. It doesn’t at all look as though it’s showing off, unlike +new Date.

As it happens, I knew about the unary + operator and optional parentheses for constructors but not the negative offsets for slice and substr, so thank you for alerting me to that.

Tim

I’m still learning myself…

There’s an annoying caveat with substr: you can pass negative indexes as the first parameter, but not the second. Fortunately slice has you covered:

// from the bowels of jQuery (removeClass)
var className = " " + elem.className + " ";
// remove named classes (omitted), and replace:
elem.className = className.substring(1, className.length - 1);

// use slice to remove single leading and trailing space:
elem.className = className.slice(1, -1);

You’ve saved us some time and a few characters with the few things we just learned from this post, haha:

1) + converts to a number (by implicitly calling valueOf)
2) default constructors require no () at the end, that’s cool
3) you can use a negative index in String.prototype.substr

We knew PHP had this functionality (in substr()), but we had no clue you could do this in JavaScript without modifying String’s prototype.

Thank you,

All my future websites

I agree – using dark magic can make code awkward to understand and hard to maintain.

However, I came across the three styles in the bowels of widespread JavaScript libraries which aim for stability and compactness. In these cases I’d expect the libraries to go with the most compact and efficient implementation.

There’s another widespread idiom that avoids deep JavaScript knowledge: do you feel the same way about this – checking the end of a string:

var summat = "foobar", tail = "bar";
// common idiom for Java's endsWith:
if (summat.substring(summat.length - tail.length) == tail) …

// using knowledge of substr:
if (summat.substr(-tail.length) == tail) …

// For hard-coded tails - compare the substring and substr versions:
if (path.substring(path.length - 1) == '/') …

if (path.substr(-1) == '/') …

I’d guess a lot of JavaScript coders don’t know they can use negative offsets in substr, but it doesn’t seem quite so deep as +new Date.