It means if variable is truthy, then execute the block. In JavaScript, the following are falsey

• false


• 0


• NaN


• undefined


• null


• (Empty String)

Other than the above, everything else is truthy, that is, they evaluate to true.

If the variable didn't exist at all (that is, it had never been declared), that could would throw a ReferenceError because it's trying to read the value of a variable that doesn't exist.

So this would throw an error:

if (variableThatDoesntExist) {

    console.log(truthy);

}

This would log the word truthy:

var variable = Hi there;

if (variable) {

    console.log(truthy);

}

And this wouldn't log anything:

var variable = ;

if (variable) {

    console.log(truthy);

}