While looking at some examples I've seen that early on in the code there will sometimes be a line that simply says "use strict"; which can be confusing to a beginner trying to understand every line. A lot of times "people will ask, what does this mean?" and kind of get shooed off with a "just do it" response. Here I'll try to explain some of what the difference is between doing it and not.
ES5 Strict Mode
Strict Mode is a boolean "mode" for ECMAScript 5 that affects how your Javascript code is interpreted. It's defaults to false (strict mode off), but by including the line 'use strict'; it is toggled to true strict mode on). Strict mode is basically a subset of normal Javascript that prevents you from using generally accepted to be unreliable features of Javascript that were never fixed / removed. The main points are:
- Assignment to an undeclared variable isn't allowed. Normally, assignment to an undeclared variable sets it in the global scope which is generally accepted as very bad so this is a great feature for strict mode to have.
- Trying to delete things that cannot be deleted throws an error in strict mode (silently fails normally).
- eval (cannot be reassigned, overridden, or used as a variable or property name and can't introduce new variables to the scope. (We've seen before how there are better choices than using eval() for general parsing).
- The with keyword is removed in strict mode (because it is seen as having many bad characteristics).
Developers don't want their underlying platform itself to be buggy. You want to know that if something is not working, it's your fault. Fix something. By using strict mode you'll be much on a much safer path with a higher likelihood for success.
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