Question

JavaScript can be made a little more strict by enabling strict mode. This is done by putting the string "use strict" at the top of a file or a function body. Here’s an example:

function canYouSpotTheProblem() {
  "use strict";
  for (counter = 0; counter < 10; counter++)
    console.log("Happy happy");
}

canYouSpotTheProblem();
// → ReferenceError: counter is not defined

Normally, when you forget to put var in front of your variable, as with counter in the example, JavaScript quietly creates a global variable and uses that. In strict mode, however, an error is reported instead. This is very helpful. It should be noted, though, that this doesn’t work when the variable in question already exists as a global variable, but only when assigning to it would have created it.

Another change in strict mode is that the this binding holds the value undefined in functions that are not called as methods. When making such a call outside of strict mode, this refers to the global scope object. So if you accidentally call a method or constructor incorrectly in strict mode, JavaScript will produce an error as soon as it tries to read something from this , rather than happily working with the global object, creating and reading global variables.

For example, consider the following code, which calls a constructor without the new keyword so that its this will not refer to a newly constructed object:

function Person(name) { this.name = name; }
var ferdinand = Person("Ferdinand"); // oops
console.log(name);
// → Ferdinand

So the bogus call to Person succeeded but returned an undefined value and created the global variable name . In strict mode, the result is different.

"use strict";
function Person(name) { this.name = name; }
// Oops, forgot 'new'
var ferdinand = Person("Ferdinand");
// → TypeError: Cannot set property 'name' of undefined

We are immediately told that something is wrong. This is helpful.

Strict mode does a few more things. It disallows giving a function multiple parameters with the same name and removes certain problematic language features entirely (such as the with statement, which is so misguided it is not further discussed in this book).

In short, putting a "use strict" at the top of your program rarely hurts and might help you spot a problem.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.