Guard & Default

mon14aug2006—33w226d61%— 03h53m00s—0utc

This is from Douglas Crockford’s Survey of Javascript (never program JS without your Crockford!). I thought it quirky at first, surprisingly helpful later. (Emphases added.)

The && operator is commonly called logical and. It can also be called guard. If the first operand is false, null, undefined, "" (the empty string), or the number 0 then it returns the first operand. Otherwise, it returns the second operand. This provides a convenient way to write a null-check:

var value = p && p.name; /* The name value will
only be retrieved from p if p has a value, avoiding an error. */

The || operator is commonly called logical or. It can also be called default. If the first operand is false, null, undefined, "" (the empty string), or the number 0, then it returns the second operand. Otherwise, it returns the first operand. This provides a convenient way to specify default values:

value = v || 10; /* Use the value of v, but if v
doesn't have a value, use 10 instead. */

Short-circuit logical operators are a well-known, simple idiom in several languages, but they can sometimes be confusing to read, specially when nested. What I want to point out here is that next time you have to go through code that uses them, try reading them as guard or default, as the case may be. You’ll grok ^EE them immediately, trust me.

Isn’t it striking, the power of names?