Adam Mark

Web design and development

JavaScript Tricks to Impress Your Friends

I recently came across some mind-bending JavaScript shortcuts in a web project I inherited from another developer. Most of these techniques are bad from a readability perspective—and none of them offer significant performance gains—but you should feel free to discuss them at cocktail parties to impress your friends.

Casting a string to a number

Use the unary plus operator to cast a string to a number:

// old way
parseInt("123", 10); // 123
parseFloat("45.678"); // 45.689
Number("9"); // 9

// ninja way
+"123.45"; // 123.45

Note parseInt() and parseFloat() are actually more versatile:

parseInt("123px", 10); // 123
+"123px"; // NaN

parseFloat("45.678deg"); // 45.678
+"45.678deg"; // NaN

Convert a date to UTC milliseconds

Use the unary plus operator to cast a Date object to milliseconds since the Unix epoch:

var date = new Date(); // Thu Aug 11 2011 10:47:37 GMT-0400 (EDT)

// old way
date.getTime(); // 1313074057920
date.valueOf(); // 1313074057920

// ninja way
+date; // 1313074057920

Testing for a substring with bitwise NOT

Use the bitwise NOT operator to test for the presence of a substring:

// old way
"Hahaha".indexOf("ha") > -1; // true

// ninja way
~"Hahaha".indexOf("ha"); // true

Wait, what? The bitwise NOT (or complement) of a signed integer is equal to the negative of that number minus 1:

~2 = -3; // resolves to true
~0 = -1; // also true
~-1 = 0; // false

This is kind of nuts, but it does save you from typing about four extra characters.

Testing for the presence of a value

Use a double NOT (or “bang bang”) to test if a variable value is not null, not undefined and not an empty string:

var name = "Adam";

// old way
name != null && name != undefined && name != ""; // true

// ninja way
!!name; // true

The first bang casts the value to a Boolean and the second bang negates it. In the ninja example above, the expression resolves to !false, or true.

Preempting a function call

Create a boolean expression to avoid calling a function if the function value is undefined:

// old way
var handleResponse = function (data, callback) {
    if (callback) {

// ninja way
var handleResponse = function (data, callback) {
    callback && callback(data);

In the ninja example above, if the the first half of the Boolean test is false or undefined, the second half is never evaluated.

Chaining function calls

Variables are for suckers! If one function returns another, you can just execute the returned function right away:

var stack = [function1, function2, function3];

// old way
var func = stack.pop();

// ninja way

Concatenating strings

The plus sign: also for suckers. Use an array to join string fragments:

// old way
var name = "A" + "d" + "a" + "m";

// ninja way
var name = ["A", "d", "a", "m"].join("");