"use strict" saves lunch and many more.

gistfile1.txt

/tmp> cat test.js 
"use strict"

a = 'b';

/tmp> node test.js 

node.js:134
        throw e; // process.nextTick error, or 'error' event on first tick
        ^
ReferenceError: a is not defined
    at Object.<anonymous> (/private/tmp/test.js:3:3)
    at Module._compile (module.js:411:26)
    at Object..js (module.js:417:10)
    at Module.load (module.js:343:31)
    at Function._load (module.js:302:12)
    at Array.<anonymous> (module.js:430:10)
    at EventEmitter._tickCallback (node.js:126:26)