Mastering JavaScript

JavaScript Primer
You switch on the strict mode by typing the following line first in your JavaScript file
or in your element:
'use strict';
Note that JavaScript engines that don't support ECMAScript 5 will simply ignore the
preceding statement and continue as non-strict mode.
If you want to switch on the strict mode per function, you can do it as follows:
function foo() {
'use strict';
}
This is handy when you are working with a legacy code base where switching on the
strict mode everywhere may break things.
If you are working on an existing legacy code, be careful because using the strict
mode can break things. There are caveats on this:
Enabling the strict mode for an existing code can break it
The code may rely on a feature that is not available anymore or on behavior that
is different in a sloppy mode than in a strict mode. Don't forget that you have the
option to add single strict mode functions to files that are in the sloppy mode.
Package with care
When you concatenate and/or minify files, you have to be careful that the strict
mode isn't switched off where it should be switched on or vice versa. Both can
break code.
The following sections explain the strict mode features in more detail. You normally
don't need to know them as you will mostly get warnings for things that you
shouldn't do anyway.
Variables must be declared in strict mode
All variables must be explicitly declared in strict mode. This helps to prevent typos.
In the sloppy mode, assigning to an undeclared variable creates a global variable:
function sloppyFunc() {
sloppyVar = 123;
} sloppyFunc(); // creates global variable `sloppyVar`
console.log(sloppyVar); // 123
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