Never. Seriously!

• Avoid security breaches
• Gain reusability

Benefits are numerous:

• Testing
• Static analysis
• Minification
• Reporting

i.e. provide missing native facilities

• Don't write your own implementation
• Reduce amount of engine-specific code
• Surprisingly, polyfills may perform better than natives

• Functions: start over!
• Component configuration: go back to Commandment 1

• Eases profiling
• Teaches functional programming (Think reusable)
• One exception allowed: in your testing tools

Use === instead. Always.

• 0 == '0' and 0 == '' BUT '' != '0'
• == does not test operands, it tests coerced operands
• === tests
  • Equality of value on primitive types (String, Number, Boolean)
  • Strict equality (same pointers) for anything else

• Mostly a corollary of Commandment 6
• They have very different semantic meanings

for (var i = 0; i < 20; i++) {
    var rand = Math.random()*1000;
    var duration = parseInt(rand);
    setTimeout(function logTheCrazyVar() {
        console.log(i);
    }, duration);
}

for (var i = 0; i < 10; i++) {
var duration = parseInt(Math.random()*1000);
setTimeout(function () {
i -= parseInt(Math.random()*10);
}, duration);
}
console.log('i:', i, 'duration:', duration);

• Makes your code more portable
• Avoids variable leaking

• Contains detailed documentation
• Provides fast, valid polyfills
• https://developer.mozilla.org/fr/