<sarc> var that = this; </sarc>

It's not the world's most powerful piece of code, but I think I like this one the most...

// Takes DOM element, returns boolean
// tests if element's text has been truncated
function isTruncated(el) {
  return (el.scrollWidth > el.clientWidth);
}

(I got this from Jason Berry: http://output.jsbin.com/ulajif/2)

Couple of reasons I like this:

• people were convinced it couldn't be done, but Jason resolved the discussion with a working test case
• it lets you add a title or tooltip to truncated items, without spamming the UI with tooltips on everything that could possibly get titles. It's the kind of polish that takes a good UI to a great UI.

Get a random number:

  var random = (min, max) => {
    if (!min || min < 0) {
      min = 0;
    }
    if (!max) {
      max = 9007199254740991;
    }
    if (min === max) {
      return max;
    }
    if (min > max) {
      min = Math.min(min, max);
      max = Math.max(min, max);
    }
    let offset = min;
    let range = max - min + 1;
    let rd = Math.floor(Math.random() * range) + offset;
    return rd;
  };

Get relative time:

    var relativize = (input) => {
      let t = input instanceof Date ? input : new Date(input);
      let delta = new Date() - t;
      let nowThreshold = parseInt(t, 10);
      if (isNaN(nowThreshold)) {
        nowThreshold = 0;
      }
      if (delta <= nowThreshold) {
        return 'Just now';
      }
      let units = null;
      let conversions = {
        millisecond: 1,
        second: 1000,
        minute: 60,
        hour: 60,
        day: 24,
        month: 30,
        year: 12
      };
      for (let key in conversions) {
        if (delta < conversions[key]) {
          break;
        } else {
          units = key;
          delta /= conversions[key];
        }
      }
      delta = Math.floor(delta);
      if (delta !== 1) {
        units += 's';
      }
      return [delta, units].join(' ') + ' ago';
    };

I have a few I always use in my applications these days. Cloning things is something I find myself doing routinely in single page applications and comparing values. I could list way more, but I stopped at three, it was hard just choosing one.

Cloning an array:

function cloneArray(array) {
    return array.slice();
}

Cloning an object:

function cloneObject(obj) {
    return JSON.parse(JSON.stringify(obj));
}

Object comparison:

For comparing if two objects are the same. Loops through and checks if their properties match, their lengths and so on.

function objectIsEqual(a, b) {
    var aProps = Object.keys(a);
    var bProps = Object.keys(b);

    if (aProps.length !== bProps.length) {
        return false;
    }

    for (let i = 0; i < aProps.length; i++) {
        var propName = aProps[i];

        if (a[propName] !== b[propName]) {
            return false;
        }
    }

    return true;
}

if i have to pick ... this is one of the most often used in my career :D .... (copied from stackoverflow ofc....)

var delay = (function(){
  var timer = 0;
  return function(callback, ms){
    clearTimeout (timer);
    timer = setTimeout(callback, ms);
  };
})();

Returns an object with information about the viewport for some responsive JavaScript goodness:

function getViewport(){
  this.w = Math.max(document.documentElement.clientWidth, window.innerWidth || 0);
  this.h = Math.max(document.documentElement.clientHeight, window.innerHeight || 0);

  this.output = {
    w: this.w,
    h: this.h,
    orientation: this.w < this.h ? "portrait" : "landscape"
  };

  return this.output;
}

I have more than a few... But here is one I like to reference: (super auto complete!)

for(i=0; i<wordarr.length; i++){

 // As I am typing words into a textbox, I loop through a list of saved values so that  (character by character) if the letters (in sequence) is contained in any word in my stored list; add to a clickable container; The clicked/selected item appears/copies into the textbox I am/was typing in:


if(wordarr[i].indexOf(text.toLowerCase())>=0){

autocomplete.innerHTML = autocomplete.innerHTML+"<a href='#' onclick=\"setText('"+wordarr[i]+"')\">"+wordarr[i]+"</a>"+"<br />";

  };`
}

Switches off all the remote stylesheets, so you can see how you page looks with only inlined CSS.

 (function() {
     var styles = document.querySelectorAll('link[rel=\'stylesheet\']');
     for (var s = 0; s < styles.length; s++) {
         styles[s].mediax = styles[s].media;
         if (styles[s].media === 'only x') {
             styles[s].media = styles[s].mediax;
         } else if (styles[s].media !== 'print') {
             styles[s].media = 'only x';
         }
     }
 })();

I wouldn't call the following my favourite snippet; but one that came to my mind after I read your question. So might as well share it here. :) When I am building something simple, and want globally-unique identifiers for my objects, the following function does it for me.

function createGUID() {
  function S4() {
    return Math.floor((1 + Math.random()) * 0x10000)
      .toString(16)
      .substring(1);
  }
  return S4() + S4() + '-' + S4() + '-' + S4() + '-' + S4() + '-' + S4() + S4() + S4();
}

var guid = createGUID();