Page 197 - JavaScript

For example, this can be used to ensure that a nullable value is converted to a non-nullable value:

var nullableObj = null;
var obj = nullableObj || {}; // this selects {}

var nullableObj2 = {x: 5};
var obj2 = nullableObj2 || {} // this selects {x: 5}

Or to return the first truthy value

var truthyValue = {x: 10};
return truthyValue || {}; // will return {x: 10}

The same can be used to fall back multiple times:

envVariable || configValue || defaultConstValue // select the first "truthy" of these

Short-circuiting to call an optional function

The && operator can be used to evaluate a callback, only if it is passed:

function myMethod(cb) {
// This can be simplified
if (cb) {
cb();
}

// To this
cb && cb();
}

Of course, the test above does not validate that cb is in fact a function and not just an Object/Array/
String/Number.

Abstract equality / inequality and type conversion

The Problem

The abstract equality and inequality operators (== and !=) convert their operands if the operand
types do not match. This type coercion is a common source of confusion about the results of these
operators, in particular, these operators aren't always transitive as one would expect.

"" == 0; // true A
0 == "0"; // true A
"" == "0"; // false B
false == 0; // true
false == "0"; // true

"" != 0; // false A
0 != "0"; // false A
"" != "0"; // true B

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